Need help with contractions...

So I feel like a freak. Since about 35 weeks or so, I've been feeling pelvic pressure on an off, often for an hour or two at a time. I was thinking these were contractions and have had that confirmed as I'm dilated and effaced. So my issue is that I've been feeling that same pressure again, especially over the last few days, and it's lasting longer than before and coming on more frequently. I can't get a handle on timing them at all, though. There isn't what I consider a definitive start/stop point. Is anyone else experiencing the same thing??? How do I know when to "take it seriously?"