Regular contractions at 28 weeks

Does anyone know why I would be having regular contractions (like Braxton hicks) but not in labour?

Yesterday I started getting my usual Braxton Hicks but they turned more regular (11 in 2 hours) with period type pain. By the night they were coming every 4 minutes and painful so I went to the labour ward to be monitored. They confirmed I was having regular contractions but I tested negative for being in labour (that fetal cells swab test) and my cervix was long, baby is fine and I was sent home after 4 hours monitoring.

This morning I felt ok but a bit sore, I went into work and they started back up again! I've gone home for some rest. What is this if it isnt early labour?