Should I be gaining weight when I start dieting?

Without seeing what you're eating, it's difficult to really figure out what is going on, but there are a few things to consider:

1. If you are not using measuring cups, spoons, and a food scale, you may be underestimating how much you are actually eating.
2. When you start exercising (or change up your routine), there is always a possibility that you will retain water, as your body uses the water to help heal your muscles.
3. Your sodium intake may be too high. Sodium contributes to water retention, which will cause you to gain weight temporarily. If this is the case, you should try to decrease your sodium intake and increase your water intake. NOTE: Many processed foods, deli meats, frozen dinners (Lean Cuisines, etc.), and restaurant foods contain loads of sodium.

Just one week may be too early to really tell. 1.5 lbs could be the result of gaining more weight in muscle than you've lost in fat, It could be just the natural fluctuation of your weight. It could be the clothes you were wearing when you weighed yourself each time. It could be a combination of several things.

If you weigh yourself once a week I would encourage you to weigh in the same time of day (I usually weigh in first thing in the morning on Sunday) wearing the same basic clothes and prior ot having eaten breakfast. This way you can remove some of those variables and have a more accurate picture over the long term.

The most likely explanation is that you are retaining water from starting a new exercise programme (or increasing intensity). Your muscles are holding onto water as they repair themselves. This is noting to worry about and will go away (but probably come and go as you increase intensity etc).

Assuming you have cut calories from what you were eating before, you have probably lost some fat, but this loss is being masked on the scale by the water retention. Just keep at it, and you should see a loss soon.

Just to give some perspective... the number of calories you would need to eat to maintain your weight is called your Total Daily Energy Expenditure (TDEE). The goal MFP has given you is less than that (to allow you to lose weight) - maybe 500 or 1000 calories less, depending on how much you said you want to lose a week. 1 lb of fat equates to approx. 3,500 calories. Therefore, to have gained 1.5 lbs of fat over the week, you would need to have eaten over your goal, all the way up to your TDEE, and then an additional (approx) 5,250 calories. I'm thinking that's not what happened.

In the meantime, make sure you are weighing/measuring/logging all your food intake accurately.

Exercise calories are there to be eaten (otherwise your calorie deficit may be too large). If you are using the MFP database to estimate calorie burn, it makes sense to eat maybe two thirds of them back, as the database can be inaccurate for some people. According to your ticker, you have quite a bit to lose, so you could get away with not eating all your exercise calories for a while, as you currently have the fat stores to support a larger deficit. Long-term though, it's better to have a smaller calorie deficit to avoid losing too much lean mass, and to avoid stalling.

You could be gaining muscle that weighs more then fat , that's exactly what happened to me good luck

LOL! C'mon now, don't troll poor OP. Or if you're not trolling, what kind of stack are you cycling?

You could be gaining muscle that weighs more then fat , that's exactly what happened to me good luck

Muscle does not weight more tha fat, a pound is a pound. A pound of apples weighs the same as a pound of cotton balls.
Muscle does take up less room compared to fat.

Claire00175, you are retaining water in your muscles, which is completely normal. When you start working out for the first time, or start a new program your muscles are being worked in new ways. To repair and grow they hold onto water. You can retain water for the first 4 weeks.

For example, when I started Insanity I gained 4 lbs during the first month BUT I lost 1 pant size, 3 inches and 4% body fat. So the scale does lie.

Just hide the scale for the first month.