Missed periods

I haven’t got my period since late March and I’m getting concerned. I haven’t been sexually active either. About a week ago I got close to having sex with this guy but he couldn’t get it in so after about 15 minutes we stopped trying. I told my mom how I haven’t gotten my period and she thinks it could be due to my weight loss. I lost about 25 since March and she thinks this could be why. But also I’m just worried that maybe I have something medically wrong with me. She wants to go to my doctors and tell them and I’m not sure if I should tell them I was “sexually active” or not because I told my mom I wasn’t. Also I haven’t got my period in months and this one night thing I had with this guy was only a week ago. I’m more concerned that there’s a medical issue involved but I’m not sure. Should I go to a doctor?