It could easily be a hundred different things. more growing, muscle cramps, dehydration could play a big role, you could be working too hard and need to take it easy, as your body has to start preparing for labor. its gotta build up its strength for it, you cant get huge muscles in one day, its the same with your labor muscles, they have to work and contract to strengthen over time. It could be something that is serious. I would suggest watching you water intake, talking to your doctor about what is going on etc.you might be experiencing Braxton Hicks contractions they prepare the body for upcoming delivery and labor pains. You may find that you sometimes have trouble with stairs or a brisk walk, or even completing a long sentence. The big concern here is that pregnancy is a time when your blood is very coagulable (clottable) and blood clots, which form in the veins of the legs or pelvis, may travel to the lungs, causing a pulmonary embolism. Pulmonary embolism, which usually causes severe shortness of breath and rapid breathing, is a medical emergency.