Im sorry to hear that you are at high risk, but thank God you made it to the third trimester! Just try not to put to much stress on your body, such as exercising and such.
Preterm labor often starts without obvious symptoms. But you may notice one or more symptoms, including:
Menstrual-like cramps, with or without diarrhea.
A feeling of pressure in your pelvis or lower belly.
A persistent, dull ache in your lower back, pelvic area, lower belly, or thighs.