My mother thinks something might be wrong with my thyroid

Here's some background knowledge before I get into my problem, I'm 17, I do have a boyfriend but we've never had sex, I'm a virgin and so is he. I'm only 5'1 and I'm about 165, I'm very overweight I know. I use to be normal size fairly skinny my freshmen year, I'm a senior now and I've gained all this weight. My mother thinks something might be wrong with my thyroid also because I'm always tired and get cold easily and have randomly gained all this weight. I don't eat a ton, but I'm not a big exerciser. I was suppose to get my blood tested for thyroid issues and some other things awhile ago, but I haven't.  I also have an irregular period, I sometimes have my period every other month. I last had my period September 23-27, it was light and had some clots. But I was sick with a bad cold at The time. October goes by no period which I didn't think much of because I miss a period sometimes and I've been super stressed with school lately. So I was like I'm fine ill get my period in November. But it never came, it's now the beginning of December, still no period even though according to my calendar I should have it in the next week or so. But I'm scared that something's wrong, I've also had really bad muscle spasms in my legs, arms, sides and sometime in my back and stomach. I'm always worried in the back of my mind that I could some how be pregnant, but that's not really possible since I've never had sex. I just want to know what's wrong, I'm slightly scared to talk to my mom because she's going to just assume the worst, because pregnant of course but I read having thyroid problems can mess up your period, and so can ovarian cyst. I'm going to talk to her this weekend and go to the doctor. But I wanted someone else's opinion on my situation thank you.