Muscle Fatigue and dehydration?

I'm no expert, and I haven't lifted regularly in three years, so I could be wrong and this may be an experience key to the program you're doing, like a wall you're just going to break through.

But it does sound like your muscles might require more rest between your workouts. Are you stretching as best you can beforehand?

Yeah, I'd go with the calories too. There's a saying, "there's no such thing as overtraining, there's only undereating", but you get the idea. Then again, it totally could be dehydration, or lack of sleep. Everything is related to recovery though. Dehydration and sleep, you know how to fix.

Undereating could be catching up with you over the 3 weeks training. You could do a few different things depending on your goals. If it is weight loss you are going for, then maintain your calories and cut lifting and cardio to something a bit more sustainable. I don't know what that looks like, but for instance, do the weight lifting routine you did last week if it was lighter, you might have to play around with that a bit. I'm not big on cardio for losing weight. I think it's easier to not eat the cookie than to eat it and go for an hour jog , but that's just me. I think you could easily do 3 cardios per week instead of 5 if you kept your 1600 calories.

I will say, most people start with 3 day full-body weight lifting and get the best results, so maybe you want to reduce that to start.

What is your lifting routine? 5 days of lifting is a LOT for a beginner. You should be sure not to hit the same muscles two days in a row. You'd probably be better off doing 3-days of full-body lifting Mon, Wed, Fri.

Hey I'm no expert, still a noob to this healthy lifestyle... but your body is clearly telling you something isn't right, try and up your calories a little or up your protein? if you're tired have a rest day? also possibly go see a doctor as dehydration and tiredness is a symptom of diabetes, great advice from previous posters also, its better to lose weight slow and be healthy than to lose weight fast and have health issues, I feel like a broken record but I worked myself too hard at the beginning and under ate and due to it I have gallstones that are so painful all because I thought losing as much weight as fast as possible was the best thing to do, I recently found a useful discussion I wished I found a few months ago: http://community.myfitnesspal.com/en/discussion/10569458/why-eating-too-little-calories-is-a-bad-idea/p1.
I wish you all the best in your weightloss journey and I hope you feel better soon
Safe paths

Sounds pretty normal to me. Extra demand on your body = should be reflected somehow initially. I would just make sure to drink plenty of water and make sure you are getting enough calories. I suppose you could increase them slightly on some days just to be sure

When my body starts wearing down, it usually means J have slacked on stretching. Stretching does wonders for me. Give it a try. Good luck.

Drink amino acids during your workout!!!