I have exactly the same problem around 2 weeks ago and constantly on paracetamol to help with the pain. Yes, like Mairi, the dentist said because I grind my teeth when I sleep and because of the pregnancy hormone, the muscle damage is more noticeable. Now I'm just waiting for my mouth guard from the Dentist which should be ready on Thursday.
I had this and when I saw my dentist he said that there was lots of signs ive been grinding my teeth in my sleep - now have a bite guard and it’s much better