Contractions or Irratated uterus?

I'm 24 weeks pregnant and was called to my local midwife center last night due to getting a few contractions.  They are not VERY painful but hurt a little and my stomach goes rock solid for 20-30 seconds each contraction.  I get these around 3/4 every hour for about 2-3hours then they stop for a few hours.

I was told that if the contractions woke me up or were getting more regular to go to my local hospital.  I was also told it could be a "Irratated Uterus" .

What does this mean for my baby and me?  Am I right in thinking there is a possibility over the next few weeks that the contractions MAY get worse and I could fall into premature labour?  And does having a irratated uterus put me more at risk?

Please Help!

[have 2 kids]

Are you drinking enough fluids?   Dehydration can cause an irritable uterus.    Couldn't the clinic tell you if it was preterm labor, braxton hicks or irritable uterus?