I was going to say sciatica as well. I've had it and it is exactly as you describe. Youch.

I would recommend a visit to a physio or Bowen therapist. Both are safe to do whilst pregnant as long as they're experienced.

If it is sciatica, pilates, yoga and swimming are all good. Sciatica whilst pregnant is not uncommon, but it can usually only be "managed" not cured. After you've had the baby you can strengthen your tummy muscles (including your psoas) which helps support your back.

Good luck!