Most likely you picked up some bad water someplace. Same thing happened to us a couple times.
If at a campground, do like already mentioned and drain, flush it a couple times. Re-fill and should be okay for rest of season. At end of season, drain, rinse inside of tank w/ a hot water wand (avail at CW) and leave anode rod out.
Next spring, re-rinse tank w/ wand, install anode rod, add chlorine to fresh tank and fill. Once full, fill the water tank w/ water from fresh tank. Let sit 24 hours, drain everything and re-fill fresh tank and do it all over again.
Doing this will remove/kill and bacteria the tank has. I Never did this before, but now I do. No problems since.