Could be waterlogged. Check your soil and see how wet it is. If it's soggy, don't water it until it dries out all the way.
If that wasn't the problem, check the pH of your water and runoff water. If those are good, onto the next troubleshooting step.
Check under the leaves for pesky critters that may have multiplied while you were gone. If there are no webs or signs of bugs, into the next step.
The lights. Did the lighting in the tent get changed while you were gone? Your plant could be trying to go back into veg.