Help. Just broke out in rash of 3 itchy raised bumps under sportsbra over collarbone. Bug bite? Shingles? (had one outbreak on back in 2003)

Itching raised rash. Localized itchy rashes suggest a local effect - exposure to the skin by an irritant or allergen, or insect bites. A metal clasp on a bra strap can cause a contact dermatitis. Poison oak or ivy, or other allergens possible. Shingles expands from an initial patch to a larger area of rash that does not cross over the midline, and is painful/sensitive. Topical steroids unless shingles (acyclovir helps).
Bumps. Itchy bumps are bug bites until proven otherwise. If shingles you would get more outbreak and it is usually painful.
Rash under sportsbra. The rash is likely from moisture under the fabric of your sports bra. Bug bite is also likely. Shingles is less likely, even with a prior history. As noted, shingles break out typically has more lesions and they are painful. As you are concerned about shingles, make an appointment to see your physician.
Bacteria. Sportsbras tend to get damp, and bacteria love warm, damp environments. My guess is the bumps are due to a gram-negative bacteria that's growing in this favorable environment. Bug bites are another possibility, as they like the same environment. Shingles is usually painful, not itchy, and thus unlikely. Change the environment (i.e. Stop wearing the sportsbra for awhile) and it should clear.
Wet. Most rashes are caused by excess moisture. Keep the area dry. Try not to lick the area. See your personal provider for further evaluation.