Ok, that PIC does not tell you if it's been a rising or falling edge. So you can either use two pins tied together, one configured to interrupt on rising edges and the other on falling edges, or do as you're doing with a single pin and look at the pin's value upon entry to the ISR. Hopefully, if it was a glitch and there's been two edges, the ISR will be immediately triggered again upon rti? (If not you've got a problem) And you'll see the timing as not ok on the second invocation of the ISR and abort?