Can you please help me find out whats wrong with my back?

Today is Monday night where I am, this began on Tuesday afternoon. I was at my college gym and got onto a rowing machine. The person that had been using it before me had left the handle bar up. When I was trying to reach for the handle bar, the person rowing to my left gave a shove while he was rowing and I felt about two small crackles to the left of my sacrum. I didn’t feel any noticeable pain at this time, nor the next day or even the day after that, perhaps my lower back felt a bit stiff when I lied down but that was it. Then on Friday afternoon I stretched my back to check for pain from what happened at the gym, I went from side to side by running my hands down my legs on both sides, hyper-extended my back and reached down keeping my legs straight. At the time of stretching I didn’t really feel pain. It was the next day on Saturday that I really noticed lower back pain in the joints around the sacrum, sometimes to the left, sometimes to the right, and sometimes both... (cont)