Mornin !!!
Defo sounds like sciatica. Be careful using heat, as with some muscular problems, it's better to apply cold. Biofreeze gel is fantastic. I know I always say this, but try and see a chiropracter. It would be counted as antenatal, so you should be paid from work.
And the worst thing with sciatica is sitting down for too long. you need to try to move around. and don't stand still on the spot, as you may find that your hip siezes and it's absolute bloody agony to step on that foot again when you try to walk. If that does happen, move you leg back and forward, taking your foot behind your and squeezing you bum cheek at the same time. I had to do this in Focus once, stood at the customer services desk. I looked like I was doing a dodgy form of ballet, holding on to the counter and pushing my leg behind me !!!

It worked though
Hope it feels better soon, and it does come and go. You may find that it just disappears when babe moves off the nerve.