sciatic nerve entrapment. keep an eye on it, if it becomes numb more than briefly/does not go away - see your gp. trapped sciatic nerve is common in pg due to the relaxation of muscles/joints/pelvic bones moving around, but if it is trapped for too long it can cause permanent damage to the nerve, so you should get it looked at for medical advice.
Things to look out for: numbness around your saddle area/genitals, trouble going to the toilet (bowel or bladder, either inability to hold it or inability to go), and if your foot droops and you cannot pick it back up - if any of these things happen, it could be cauda aquina syndrome (rare), which is loss of function of the nerve roots below the end of the spinal cord, which services those areas mentioned. If you have these symptoms, you need to go to A&E to check yourself out.
It is worth mentioning that the above symptoms for cauda aquina is most likely if you have a lumbar disc related injury, so if your symptoms are sciatic pain due to things moving around while pg, you will unlikely (probably not likely at all) get those symptoms, but i mention it in case your symptoms are in fact related to a back problem rather than pg.
Despite what the medical professionals tell me re back pain/sciatic pain and keep moving, the best thing I find for acute episodes is to rest on my back (if you can!) and take paracetemol (cant remember if you can take ibruprofen in certain stages of pg, if you can, take this as anti-inflammatory and will help lots).