Any advice? I'm scared and FTM

I should have called my doctor earlier, but I wasn't quite sure what I would even say. I think I'm freaking myself out.

I had a busy weekend and I am exhausted. Last night while getting ready for bed I started to feel extremely lightheaded and thought I was going to pass out. This morning I was driving to work, and all of a sudden started getting sharp pains in my lower abdomen, which were a mixture of feeling like I have to throw up and also go to the bathroom. I'm constipated, but I've tried to go at least 5 times today with little success. I also don't know why I feel so sick - I think this might be the sickest I've felt my whole pregnancy, and I'm just about 12 weeks.

I just feel off...I've been trying to drink water and stay hydrated, but I have an ultrasound tomorrow and I'm TERRIFIED - I've waited so long (well, you know, about three months lol) to almost be in the clear after worrying about every ache and pain (or lack there of). I don't know if the mental part of going tomorrow is getting to me, I'm just really afraid they're going to tell me something terrible. I don't know what to do - why do I feel so sick?