Normal cramps vs Pre-Term Labor?

Hey ladies, I need some guidance! I'm 24 weeks and have had a crampy/achey back for a few days, then had my first Braxton Hicks contraction, too. I know BH are normal in the second trimester as long as there's not more than 6 per hour and that this is probably just part of the baby growing ...but how do I know when I should be worried? I'm very hydrated so that's not the problem. I didn't expect to be "practicing" for labor so soon...is this indicative of pre-term labor? Thanks for any advice!