I've been trying to lose weight for 2-3 weeks and I'm starting to doubt everything now.  My weight keeps bouncing around and I just feel like crap.  
I tried the 30DS.  Level 1 was really hard on my knees and I didn't feel like it was well balanced.  I enjoyed Level 2, but at this point, my knees can't handle it.  The jumping jacks hurt my knees sooooo bad that I can't finish.  I had to stop doing it because even though I felt awesome, my knees hurt so bad that I could barely even walk.
I have also been walking and running on the treadmill.  I am pretty satisfied that I am able to run longer and longer.  I tend to only do about 30 minutes or 2 miles.  Is that really long enough to burn off fat?  How long should I aim for?
I have been trying to eat healthy.  I cut out soda (which is causing horrible headaches that blur my vision and make me nauseous!).  Only eat out about once a week now.  Try to limit my calories to 1300.  Warning TMI:  I just feel like crap because I can't poop.  I do it maybe once a week and even then, it doesn't seem like it is enough.  I'm eating a balanced diet with enough fiber.  It's hard to exercise when I'm bloated and miserable because of this.
I really don't understand how many calories I should be eating.  According to the MFP BMR calculation, my BMR is 1,596.  I'm 5'4.5" and 187 pounds.  I sit at a desk for about 4 hours a day and spend the rest of the time walking around campus or sitting in class, so I guess I would call that sedentary.  I would like to get down to 145 by this summer.  I know it will be tough, but I have to have huge goals in order to accomplish much.  How many calories should I be eating?
Sorry this is so long.  I just feel like once I can start feeling better, I will be able to work out on a more regular basis.