I feel u.. this is my 5th pregnancy and im 16 wks & feel very tired but then again I have 4 children & one is 6 months old so im always on the go..ever since I started working out I've had more energy, not the first couple of dats though cause body will get soar...but try it maybe it can work 4 u as well..:-)
Although it is very normal to be exhausted during pregnancy, you could get your thyroid hormone levels tested to make sure they are normal if you haven't already. When they are not in the normal range you will feel a lot more tired and just worn out. My thyroid levels are a little low (TSH levels are high) and the doctor raised my Synthroid dose to fix this, its a very easy fix but many women do not know they have a problem... I am 16 weeks pregnant and the other day I was doing a garage sale and by 1:00 i was exhausted and felt like I couldn't walk, all I could do was lay down, and I believe my thyroid levels had to do with it. So they can check your thyroid hormone levels next time they draw blood if they haven't already.
This is your third pregnancy, things will start happening sooner than you're used to, unfortunately. I'd just try to take it easy as much as you can. This is my 2nd pregnancy and I hit a baddd few weeks around 16-20 weeks, I felt MISERABLE, sore, uncomfortable, and it passed, so hopefully it does for you, I'm 28 weeks and other than bending over it's been pretty comfortable!