What is going on with me?

I'm 21/Female and I have been stressing out for a while because I feel like my body has been acting strangely. First of all, I hardly poop (maybe 3 times a week) and usually feel constipated or that I didn't get all of it out (whenever I do). Second, I am always tired no matter how long I sleep (normally I sleep 6-7 hours though); It's always hard for me to get out of bed and I'm still yawning all day. Third, I'm almost always hungry. I normally don't eat breakfast unless I have to time in the morning (if I do, it has been one Activia which is about 110 calories), then I eat about 500-600 calories for lunch, and then whatever is made at my house for dinner (and I don't really overeat either; I eat till I'm full and then maybe a BIT more if it's really good). I usually have a snack either between lunch and dinner or after dinner, but it would be something small. I usually drink a bottle of water during the day, and maybe another 8 ounces after I get home from work. The Activia works for the most part, but my problem is that I don't always have the time to have one every single day. Also, I started exercising early June.. I usually used our elliptical 4-5 days out of the week on average for an average of 30 minutes (burning anywhere from 500-1000 calories according to the machine). If I wasn't able to elliptical, I will try to compensate by taking walks, riding my bike, or swimming for about 30 mins to 1.5 hours depending on the day. For the first week I lost almost 1 lb a day, and then it stopped. I have been doing about the same schedule since then. I am not losing weight yet I feel like I'm eating less than I used to and exercising more. Right now I stand at about 5'8" and 140 lbs. I do take Aviane medication. Can anyone provide some insight on what may be going on with me?