Contractions,,,,,,,,,,,,,, What the heck

I was at work last night and started having contractions. They were not very painful so I came home this morning and went to bed. When I woke up this afternoon I started having more contractions and they were getting stronger and closer together. I took a bath and drank lots of fluids and they still didn't go away so I went to the birth center to be checked out. I was having contractions about every 3 minutes apart. They gave me a medication to stop the contractions and checked my cervix. I was not dilating thank god but my cervix was starting to thin so I got an ultrasound. I know I have had more ultrasounds than anyone ever, well that is how I feel. The ultrasound confirmed that my cervix was about 50% effaced. My contractions slowed down after 2 shots of the medication so I got sent home and was told to take it easy and to monitor and come back if the contractions came back, So that is what I did, I came home and went to bed.