Could I deliver early?

I'm hoping to get some advice and read some stories from experienced moms. I'm expecting my first child Oct 17th. My doctor has said that my baby boy is on the money on his growth chart and that the US I had at 19 weeks baby was in the 50th percentile. Recently, I had a day were I had a strong feeling of him being extremely low with sharp pains in my vaginal area as well as feeling swollen and sore but no contractions. It also felt like his head was right at the opening. The night prior I had some BH but nothing serious. I haven't had the strong sensations like that before and haven't had them since in that intensity. I do feel him a lot lower and my belly has dropped. I spoke with my doctor and he seems to think everything is normal and no need to worry. He will have me scheduled for an US as well as vaginal exam at 36 weeks. While I do respect his medical standpoint, I was wondering if other moms have stories, warning signs etc that they could share about early babies and what things signaled you that baby was coming. I'm a bit worried now that he's going to come earlier than his due date. I'm 33 weeks today.