Right side pain at 23 weeks (round ligament?)

Megan • I am pregnant with our first child, a boy, due May 31st.
Has anyone else experienced this? Two nights ago I started to have really bad pain on my lower right side. It kept me up all night and persisted during the day although the severity would go up and down. It feels sort of like a cramp, but really like someone is reaching in and squeezing an organ there. I tried Tylenol and it didn't seem to help. Warm compresses relieve it a bit but it always comes back!
Is this round ligament pain or something else? I thought round ligament pain was intermittent and this is constant. Do you think I should be concerned? I've had no other symptoms or spotting or anything.
Any suggestions on stopping the pain or preventing it?