Dehydation, hyponatremia , hyperthermia, trisma and bruxia are probably the most common problems. So keep yourself hydrated with drinks containing electrolytes (Gatorade for example) but don't drink too much. Hyperthermia is over-heating and this shouldn't be a problem unless you're too physically active (ie: dancing for long periods of time) or in an overly heated room or environment. Trisma and bruxia is jaw tension and grinding which is uncomfortable and if severe could wear down your teeth. Magnesium and calcium may help reduce this side effect, taking a complete vitamin supplement in any case certainly wouldn't hurt. Having gum, a tooth pick or other object to chew on also helps. As does placing the tip of your tongue between your teeth (you wont bite yourself).
Nystagmus or involuntary eye movement may also occur at higher doses, this can cause double vision or make everything look like you are under water but is actually pretty cool.
There is a very low risk of severe negative reactions if the pills do in fact contain MDMA. Some pills also contain other MDxx chemicals which are similar to MDMA in chemical structure and effect but not exactly like it.

Again there is no telling what is in the tablets you have, here is a briefing on the most common adulterants.