Especially if you've done one of the killer strength classes, you may experience what is called delayed onset muscle soreness. Physical exertion (e.g. training or exercise) can cause microtrauma (small tears) to the muscle fibres (the pain you may experience). Soon after, your muscle will adapt to prevent damage, hence the soreness if the exercise is repeated. This is a natural process of muscle building.
If you do not experience improvement from your pain in seven days, we recommend you consult a physician.