What's been going on with me?

After shedding about 24 pounds in the summer, in 4 months, I'm nearly back to where I was in the first place! From 176 to 152 in 3 months and back to 165 in 1.5 months! Yes, I' might have been eating a little more, especially considering I'm far away from home, in college trying to survive on my own. But not definetely as much as leading to 13 pounds gain in 40 days. The funny part is, apart from my butt ( which once upon a time was my smallest part), it is nearly invisible! I still fit into my clothes. I still keep my dress size. Yes I admit, they might be a little tighter but does not matter i still rock them! Only 3-4 people could tell me I've gained weight, others did not even notice.
And the scary part is. I can' lose them this time. Whenever i realized i've gained weight, i took my calorie intake under control. I especially tracked my carbonhydrate intake and limited it. There were subsequent days when i took no more than 1200 calories but at the end, let alone losing some, i even gained a little more. It especially happened when i saw 160 on the scale. After 5 days of resisting temptation and making definetely healty choices, I was up to 164! I really can't figure out what's wrong with me? There's something wrong with my leptin or other fat-burning hormones, I think. I'm going to have a blood test done tomorrow morning just in case but nevertheless i need your help as to what i can do to avoid more gain, and even losing, if possible. Now I'm at home for 25 days where I can eat healthily and do more exercise but I'm afraid it won't work this time so i may not even bother it. I don't know. I'm really lost and confused. I had never been there in my life.
Also, another explanation I can think of is that, my menstrual cycle has always been irregular. Once in 2-3 monts, I mean. But for 2 months, it's perfectly regular. It occurs without even missing one day. There must be an increase in my estrogen levels which led me gain weight and hold them in my butt and nothing else is wrong. But I might be being optimistic, I don't know. And, some of you may think it's water weight or i've gained muscle. Neither is true, unfortunately. My weight has been the same for 20 days and I did nothing to gain muscle during this time. Oh, by the way I'm 5'10 and 19 years old. Thanks!