A phenomenon that can be found in younger individuals is something called accommodative spasm. This is a situation where the eyes spend so much time focused on near objects that the muscles that allow that near focusing start to spasm, which can cause headaches and difficulty changing planes of focus, such as when you're driving.
Often times people with this problem benefit solely from a dilated eye exam (the drops used to dilate the eyes can sometimes break the spasm). A refraction (test for glasses) should be done after your eyes are dilated and this can sometimes reveal a true glasses prescription which is less myopic and more hyperopic than what you're currently wearing because the accommodative muscles have been spasming for so long (it can simulate more myopia).
I'm not sure if your doctor saw that your eyes turned in excessively when you read and that's maybe why you have prism in your glasses, but if you do have some accommodative spasm and there is not a significant change in your refractive error even when measured after you're dilated, then putting a bifocal segment in your glasses may help with this discomfort and your visual symptoms.
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DISCLAIMER: My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must be examined by your doctor.