I highly recommend taking it easy for a few days then seeing a chiropractor or physical therapist who practices Active Release Technique (ART) to get it unglued. It's hard muscle to deal with for many reasons. It tends to get "glued" to your quadratus lumborum (muscle in your lower back) when it gets really tight. When it's tight, it will also pull on that area in your mid back where your hip flexor, QL, lat, diaphragm, and lower rib allow share proximity. Avoid ab exercises - you are probably working your hip flexor more anyway.

I didn't realize how bad my hip flexor was for years. Really caused a lot of lower back pain, and I went to many MDs and PTs who had no solutions and just made it worse. I finally went to chiropractor who practices ART, and she helped to start getting that muscle under control.