I'm not allergic to watermelons ove eaten it a binch but the last two times i've at it the roof of my mouth itches and so do the corners of my lips why is that?

Oral allergy syndrom: You likely have hay fever as well and the reaction to watermelon is likely from cross-reaction to ragweed allergy. Avoidance is the best approach but on the other hand this type of reaction does not usually lead to throat swelling.

OralAllergy Syndrome: Oral allergy syndrome may occur in up to one-third of individuals with seasonal allergies and results from a cross-reactivity between seasonal airborne pollen proteins (i.e. tree, grass, weed) with similar proteins that are found in various fresh fruits and vegetables. Common symptoms include itchiness, tingling and/or swelling of the mouth, tongue and throat, immediately after eating.

Ragweed: Could be cross-reactivity with pollen allergy to ragweed, called oral food allergy syndrome. Not unusual. Other fruits/raw veggies might start bothering you, too. If your pollen allergy worsens and you end up on allergy shots, many times the fruit issue will improve. Blood allergy tests can miss this kind of food allergy; often skin tests by an allergist with the fresh fruit are needed. See one.

Food pollen allergy: Food-Pollen Allergy or the Oral Allergy Syndrome is what you are describing. Most likely you are allergic to some pollens like ragweed and there is an allergic cross reactivity. Fresh melon is more likely to cause it.