I don't think it is watermelon, it may be that eating may have caused pressure in that area again. I would not eat for four to five hours and see if the pain goes away.
Is your sciatica pain on the left leg?
I do not know much about sciatica, but two things come to mind, yoga, especially child pose, a good chiropractor and other thing you can do is take Epsom Salt baths at least once a day. You can a,so take turmeric powder with honey twice a day to reduce any inflammation you may have.