Unless the shop was VERY familiar with your bow, or the cams are marked somehow, I find it hard to believe they can say your string stretched. Possible, but most likely without one of the above its a guess.

anyway, to "fix" it, do what bigbulls said above, need a press.

If it did stretch the effects would be longer draw length, higher draw weight and the nock point would have moved slightly.

I would suggest taking a few shots and see for yourself if anything is off.

--Bob