I agree with the others. It could also just be the babies position, but always better to be safe then sorry. I would just give your Dr. a call and see if they want you to come in. Wow, 36 weeks....time sure flies. I cannot wait to hear the birth story!!
Call your doctor or if it gets worse go to emergency room. Hope everything is okay
If it continues, or becomes really worrisome to you, it wouldn't hurt to call your doctor and let him know what's going on. I had this for the entire third trimester of my first pregnancy, and it was discovered through a sonogram that I have a leaky hole in my heart that was causing me to be short of breath and over tired due to the extra work it was having to do to support the baby. But my case isn't typical - your baby is probably just wearing you out! Best wishes and hang in there, you're almost done!