What's causing these itchy, painful bumps on back of knees? i ate a cherry the other day which made my mouth very itchy and face heated up. allergic?

Allergy: You may have had an allergic reaction the other day. Most likely it was from the cherry. The bumps could be from the allergic reaction but I cannot see them very well on the photo. I suggest you see your doctor for confirmation. In the meantime over the counter Benadryl (diphenhydramine) will help with the itching. Be seen immediately for any lip or tongue swelling, lightheadedness, or trouble breathing.