Missed period, gaining weight, negative test?

Ciara

I’m on birth control and have been since August. I was two hours late with a pill one night but took it as soon as I realized. I was supposed to get my period two weeks ago but nothing. I’ve taken two pregnancy tests, one when I was supposed to get my period and one a week later and both were negative. I feel like I’m gaining weight which is stressing me out when only a week or two ago I was feeling confident and thin. What is going on? Should I be worried?