J., I am 15 weeks pregnant with #4 and have been very tired lately, too. Like the others said, fatigue is normal with pregnancy, but I urge you to ask your doctor/midwife for a full panel blood screen. I have the problem of extremely low ferretin which is your iron stores. This is not usually tested for with a routine blood draw, especially if you don't show up anemic (that's low hemoglobin and/or hematocrit). You could start on iron if you need it, and you would feel a difference in a week. (IF that is the case, I recommend ordering a brand called Thorne Ferrasorb which is iron capsules that are much more readily absorbed than those black tabs from the drugstore.)
Aside from a health deficiency, just lay low, eat well - baby needs a lot of healthy fats, calcium, and iron - and don't expect too much of yourself. Ask for help now, knowing that you can repay your friends and family in the future when you're doing great again and they need the boost!