Is it normal to feel weaker?

No, it's not normal. Something is wrong.

Are you eating enough and frequently enough?

Do you have rest days from exercising?

Do you have high blood pressure? Sometimes with weight loss people's blood pressure drops and they need to have their medication adjusted because their BP becomes dangerously low(basically, their BP meds are now at too high a dose).

For us women, it could be hormones. For example, you might be pregnant, or if you're irregular, it might be your TOM coming up.

Dehydration and electrolyte imbalance can be really bad, too!

Are you eating enough to compensate for all that running?

If you're someone who is totally opposed to eating back earned exercise calories, you should at very least bump your activity level up to moderately active to compensate for all that running.

i've found I feel really bad after a few days if I don't get at least 20% of my calories from protein and 30% from fat. Given that you're drinking a lot of water (more than 12 cups a day, hopefully), and getting enough calories and enough carbs, that's where I'd look first. A LOT of vegetarians feel bad because they don't get anywhere close to enough protein.

Personally, I felt MUCH better when I quit vegetarianism-- the added protein REALLY makes a difference when it comes to being tired and weak and irritable. Try to get 75-100g of protein each day, see if it helps. Have a protein shake if you don't want to eat 8oz of meat on top of what you normally eat.

Are you sticking to a 1200 calorie diet or more? It's possible you aren't getting enough to eat. You shouldn't feel easily fatigued if you're getting enough calories and water... Your body needs fuel to give it energy. Is it possible you're not eating enough?

Protein is extremely filling, and lasts for hours and hours, though I don't like protein shakes and try to get my protein from meat and eggs and dairy and nuts. As for fat, one good way is to stir fry your veggies instead of steaming them. A tablespoon of olive oil gives you like 100 calories of fat and with a couple cloves of garlic and some black pepper can flavor a chicken breast and a pile of delicious vegetables. Altogether you're looking at less than 500 calories for a meal that will seriously keep you full through your entire evening. Also, nuts are high in fat and have quite a bit of protein. Almonds are a little lower than fat and higher in protein than cashews, but I usually eat my favorites anyway. Cheese has a surprising amount of protein, in addition to its fat content. Salad dressing usually has significant fat content (I like Annie's Tuscan Italian). Eat vegetables to make up for the lack of volume, you should be good, so long as you put down at least two cups of water each and every time you eat something. For breakfast, I'll throw some mushrooms and broccoli into a little pan with a pat of butter, then when they're getting close to done I'll crack a few eggs and some green onion into it and finally an ounce of sharp cheese (parmesan, cheddar, feta). 550 calories that will get you through your entire morning no problem. If you're doing exercise you should have enough calories to eat that a couple 500 calorie meals shouldn't be a problem for you. If I'm down on protein but can't have a nice meal like that, 200 calories worth of deli turkey and some celery for volume makes a damn fine snack. Actually, you can throw deli meat into all sorts of stuff to up the protein, from salads to that delicious egg scramble. And eat beans! Beans are awesome sources of fiber and they have quite a lot of protein, as far as vegetables go, so they'll keep you full longer than other carbs and won't mess with your glycemic index too much. Rinse the heck out of them if they're canned and change the water several times if they're dried, it will cut the gas a lot.

One other trick is to down 2-3 cups of water if you're having a protein or fat snack, trick your stomach into thinking you're full. The food itself is so heavy that the water should be all you need to stay full and energetic for hours. Also, I've found that by choosing foods that don't give me a blood sugar spike, my issues with feeling hungry have really diminished-- more than just feeling good and strong, that's one of the big reasons I get so many of my calories from fat and protein.

I'm definitely getting my 1200 calories almost every day, and some days are over calorie.

How is it possible to eat enough fat and still eat enough to satisfy throughout the day? I've just been focusing on eating when I'm hungry, but making each meal and snack as low-calorie as possible in order for me to be able to eat more and not be hungry.

I have some good vegan protein powder made from organic vegetable and bean sprouts that is awesome, but smoothies are high in calories and not very filling (for me).

I will try to get more protein. My muscles feel weak the last few days.