Pregnancy exercise ball?

I am 34 weeks along with my first baby. I been having some really bad lower cramps that won't go away until I am laying down on my bed. I was thinking about buying a pregnancy ball to help me. But I'm not sure if it's safe to use at this stage of my pregnancy, People told me I can go into early labor if I use one. Is that true or not? I don't want to have an early labor. Help me please!!