When I was pregnant with my first child I think I slept the entire first 3-4 months. I don't know why I was so sleepy all the time. No morning sickness or anything, just sleepy.
My second child, I didn't have that, but it was smell that really made me sick. I couldn't stand the smell of my own husband, LOL. Anything fragranty or perfumy would just make my stomach churn.
I would think your o.k., but make sure you mention it to your OB/GYN just to be sure.
Congratulations and I wish you and your family and your new little one all the best.
Not to discount any other health issues you may be struggling with, but from having had three "normal' pregnancies, I can assure you that the need for sleep at 15 weeks is perfectly normal!
With my second baby, I had to drive over an hour to my Dr's office. Well, that's not a big deal for most people, but for me .... it was horrible! I would have to pull off the road and take a little nap to be able to make it the rest of the way.
You will probably start feeling better and regain your energy in another few weeks. So for right now, keep taking your vitamin supplements and sleep when you feel the need. I know... that's easier said than done, but try to do as much as you can.
Good luck with the rest of your pregnancy! Try to enjoy the changes that are going on inside of you! It is certainly a blessing to feel the first signs of movement!