yep it's sciatica. It happens when the sciatic nerve is trapped.
I can either be as a result of hurting your back and is thus lumbar disc related, or it could be that your baby positioning is pressing on the nerve area, or as mentioned related to sacro-illiac joint problems.
It hurts in that location because the nerve runs out of the spinal cord through the sacrum and out of a foramal (sp) opening in your sacrum, which is exactly in your bumcheek! in then runs down your leg.
Agree worth talking to your Gp to rule out any underlying cause such as a lumbar disc problems. In the mean time:
(thank thumbwitch for this fab tip!) massage DeepHeat into the butt cheek, about 2 thirds down from the top of the pelvis (illiac crest) which is where the nerve comes out (presuming of course you can use deepheat in pregnancy, make sure you check), you will probably know where to rub as this is where the pain seems to start from.
Use an ice pack for 20 mins, followed by a heat pack for 20 mins, every two hours in the same area. If you have back pain, use it where there is back pain.
Lay on your back (on a cushion or towel, not on the sofa, on the floor) and gently bring your knees towards your chest, rock very slightly back and forwards, side to side, bring your legs down again. Do this every couple of hours about 5 times. You can bring your legs up together, or one at a time, whatever is the most comfortable for you, and bring them as close to your chest as you feel comfortable, that might not be too far, that's ok.
Have a warm bath.
keep an eye out for the following:
numb foot/leg/foot droop (if you can't pick your foot up)
weakness in the leg
loss of bladder or bowel control
numbness in the saddle area (the area you would feel if you sat in a saddle)
If you have any of those things, get checked out at A&E. (It is highly unlikely anything relate to something called cauda equina syndrome which is where a disc for example presses on the bundle of nerves in that area as it is very rare but a medical emergency if it does happen