While what you describe is not a very commohnly reported side effect of Risperidal, anything is possible. How long have you been on the Lexapro? Lexapro can sometimes cause headaches as an initial start up side effect. The headaches will usually subside within about 2 weeks of starting the Lexapro. If you've had this symptom for 7 months, I'm not sure it could be either of the meds.
My advice would be to let your doctor know how you're feeling. You may need some adjustments to your medication regimen. Good luck!
Have you explained this symptom to your doctor?