Thank you for your question and welcome to Just Answer.
If you are sure that the pain comes on from eating just sweet things and not from chewing in general, then this is indicative of an active cavity or a filling that is breaking down and starting to leak. The latter will not show up on an X-ray. Older silver fillings are classic culprits of this kind of pain and only need to be at the initial stage of breakdown to initiate pain. Marginal tooth colored fillings can elicit the same kinds of pain, but more so with silver fillings.
The bacteria in your saliva that causes decay will congregate in the gap between the corroding filling and the tooth . When these bacteria are exposed to sugar, they metabolize it and convert it to an acidic material which, in turn, percolates into the deeper layers of the tooth where the nerve receptacles are, causing the pain.
You need to have your dentist take a closer look at your old fillings and replace the ones that show signs of breaking down.
I hope this answers your question. If you would like to discuss this further or have any additional questions, please reply to my answer and I will get right back to you.
Sincerely, XXXXX XXXXX DDS