Sun exposure is not recommended after surgery but if it already happened there isn't much you can do. It probably did not damage the grafts but if you are concerned follow up with your doctor for a recheck.
Don't worry, you are far from the only one who has done this. I had a patient of mine only a month post op and decided it would be a good idea to play tennis all day without a hat. The result? A second degree sunburn complete with swelling and massive redness and peeling. Did it injure the transplants? No. Luckily it did not. I wrote an article about this in the 90s for our Societies journal "The Forum"Hair grafts cannot protect themselves like normal skin by darkening just after a hair transplant. It takes months for the grafted tissue to reacquire that ability. So until then it is best to not expose it to the sun. I don't think your exposure is anything to worry about. As long as the skin still looks normal and not "sunburned" you should be fine and so should the grafts.
By third week after your hair transplant procedure the follicles should have been set. However if you indeed have sunburn, you need to follow up with your doctor to get the proper treatment to make sure it doesn't lead into an infection.
There is nothing you can do about your sun burn other than keep out of the sun or wear a hat. The grafts themselves, have a skin that came from the back of your head and never, never were exposed to sun so this skin is easy to burn.