Daniella, I personally wouldn't use the heat for any more than 10 minutes at one time as Mrs D said & my doctor also said the same, but i think it's safe to reapply it every 2 hours as the heat dilates the blood vessels of the muscles, it also increases the flow of oxygen & nutrients to your muscles, also the heat stimulates the sensory receptors in the skin & should help decrease transmissions of pain signals to the brain.

But on the other side of the coin some people find alternate heat & cold applications work better for them, others find cold application works the best, which makes good sense too, as the area is already inflamed and the coldness should help to reduce the inflammation.

I hope it settles down for you soon,
Brian