Am I pregnant??? 20yr female

The last time I had sex was Dec 7, unprotected but I’m on birth control (have been for years) and he didn’t finish. I have been getting my periods regularly but they are lighter than normal, I stopped taking birth control dec 16-Jan 8 (started a new brand) and still had my regular periods but they have been lighter than normal. This past month I have gained about 20lbs and my boobs are growing like crazy I’ve always have really small boobs but now I have stretch marks on them. I took a pregnancy test on February 23rd and it was negative. But idk how tf I’m gaining weight bc I haven’t changed my daily activity’s other than the fact that I have been more active and eating healthier so it doesn’t make sense. I also started anti depressants (amitriptline) over the summer maybe august, but switched to lexapro in November is that the reason for the weight gain and crazy emotions? Idk please help me