I got an itchy rash on my vagina after receiving oral sex. What could this be? I don't have any discharge, & it's only where I shaved.

Razor burn. If it's only where you shaved, it's likely razor burn and unlikely to be related to the oral sex, although the open skin makes you more susceptible to contracting stds like herpes. A topical aloe or itc cortisone should help, if no better in 1-2 days, see your family doc for eval and treatment.
Irritation rash. Your rash is likely related to tiny scratches or abrasions caused by shaving and possibly by your partner during sex. If your partner has a beard or needed a shave your are more likely to get a rash. Ordinarily this will clear up without specific therapy. Washing with soap and water could help.