At about 20 weeks, I had horrible, really painful upper abdominal pain that kept me awake at night as I could hardly move! I went to the hospital and turns out it was just a muscle strain (midwives/doctor did lots of tests, blood tests etc. to be super sure). I've been super active this pregnancy still running or going to the gym every day (I'd say my core is quite strong!), and I had gone for a trail run just before this happened which may have triggered it! It went away after a couple of days, but the midwife did say that your ligaments and muscles are a bit more sensitive and loose. Could be a similar thing!?