One possibility is that you have a sensitivity to sunlight, called photodermatitis. This is a condition in which inflammation of the skin is provoked by the sun. It is usually not very severe, although it sometimes can cause significant itching, redness, and scaling. The solution to photodermatitis is, of course sun avoidance, wearing good sunscreen, and protective clothing. If the redness persists, a small amount of topical steroid cream may be needed to get it under control.
Another possibility would be a sensitivity reaction to sunscreen lotion or some other product that you applied. If this is the case however then you should also have some rash in other areas of the body where you applied sunscreen.
Another possibility would be 'heat rash' which is a condition in which the sweat gland ducts become clogged by dirt and perspiration, leading to small red bumps that usually do not itch and are not painful. This usually takes care of itself over a few days, but you can try gentle skin cleansing to speed up the process.
If the rash does not resolve or if it worsens, you should talk to your primary care doctor
who will help you figure out what is going on.