your body reacts to the fruits because they've got chemical similarities to the grasses- basically your immune system thinks the melon and tomato are grass- or close enough to them for it to freak out over. even if there is no grass near you, the defence system in your body still remembers allergens, so would be going around carry the message 'grass is bad- freak out when you see grass, protect cyrillia from grass invaders!', and then when melon or tomato goes into your body, it'd think 'melon=tomato=grass, sound the alert, freak now! freak now!'.
the immune system is a funny thing. it'll always recognise an allergen, but how strongly it reacts is dependant on a few factors. my reactions to tomatoes are worse (or feel worse) when i already have hayfever, cos my immune response is already triggered and releasing histamine, and its like another stressor added to the pile- similar to how a little bit of dust might make me sneeze a little, but a little dust, a few cats taped under my nose, and some trees swooshed in my eyes (or just big old pile of dust) would be pushing it too far, and i'd be reduced to a dripping, puffy faced snivelling wreck (lol, attractive). there is also a limit to how much histamine a set dose of antihistamines can handle- mine can handle me sweeping the floor of 3 days dust, but 3 weeks worth of dust invading my nose all at once is way too much for them.
it might be worth you doing a food diary. you could write down what you eat, the times, and how much you itch. perhaps don't eat tomatoes and melons in any form (ketchup, pasta sauce, etc) for a week or so, then try again and see what happens. you could try some cooked tomatoes, and compare it to fresh ones eaten on another day (sometimes with some fruits and veggies cooking breaks down the allergens enough that you don't react-i'm generally ok with tomato pasta sauce, but a tomato fresh off the plant will really hurt me). you might find a clear corrolation and track things down further- it can be pretty insightful. just be careful- apparently oral allergy syndrome doesn't tend to get dramatically worse- but there is always the risk that it could.
i'm thinking your results may well have been different had you not had the antihistamines in your system. if you had a scratch or patch test for example, your body would make a little itchy bump (or a flipping massive red weal if it was really keen) if an allergen dropped/scratched on or injected under your skin caused a histamine response. if you were taking antihistamines, that little bump might not have come up, because the antihistamines were blocking the action of the histamine on your skin tissues. if you had blood tests though, i'm not so sure... they measure igE (an antibody) and your antihistamines wouldn't do anything to the antibody production, as far as i know.
i'd have another chat to your doctor, and see if its worth going back to the ent or immunology folks. can't be fun being itchy like that all the time. i know it drives me nuts (i itch head to toe- but at least i can reach mine to scratch!) and i'll be going to the canadian docs about it as soon as i'm covered for it. at least if you know its tomatoes, you can stop your throat hurting by not eating them and getting yourself some nomato ketchup
(they do baked beans in nomato sauce, and nomato pasta sauce too!) instead!