Swollen labia after rough lesbian sex. how do i protect myself?

Gentleness & lube: A water based lubricant, a gentle and mindful partner, and "clean toys" all help to protect you. Washing vibrators after use with a mild soap solution or rubbing alcohol, never inserting a "toy" from the rectum into the vagina without cleaning, and the use of lubricated condoms over vibrators are all helpful.

Slow down: You might consider applying vaseline to protect the skin or just not go at it so roughly. Usually decreased lubrication is the problem and a product like astroglide tends to help with that.

Be gentle: Be gentle, assess yourself for possible allergic reactions, stds, and use appropriate lubricants.