If you have lots of itchy or burning bumps on the lower leg, the odds are that it is Polymorphic Light Eruption (PMLE). PMLE is a type of allergic reaction to a sudden high dose of UV radiation, especially on the legs, which tend not to be sun-exposed as often as the arms and face.
Dermatologists like to say that there is a 4F factor for PMLE: persons most at risk for it are Female, 40, Fair, and Fat (not necessarily obese, just somewhat overweight.) The bad news is that once you get it, the chances of getting it again go WAY up.
The best way to deal with it is prevention. Most drs. recommend "hardening" the skin of the legs with gradual exposure to UV A *and* B for several weeks leading up to the anticipated high exposure. This can be done by sunbathing for short periods or using tanning beds, again, about 10 minutes at a time. If that isn't an option, slathering on a broad-spectrum sunscreen every hour and taking an antihistamine before going out in the sun usually help. I take Claritin, myself; I haven't had it happen again since I started taking it before and during vacations in very sunny places.
Once you get it, the best treatment is an antihistamine or steroid cream. It is NOT the same as the sort of heat rash one gets when skin folds chafe together; using things like talcum powder on it may actually make it worse.