At 33 weeks, it is quite possible that the baby is starting to move, possibly even starting to engage.
Nina arrived at 37 weeks/2 days and she was posterior. I'd had a check up with my GP 2 days beforehand, and she wasn't posterior at the time.
Like RoryRory said, the back ache is on par with contractions. So you're getting it from both back and front! I caved in and had an epidural, and it bought enormous relief. An hour after the epidural went in Nina was born. It was 9 hours in total.