I have ADHD and was on ritalin (methylphenidate). But every time "that time of the month" rolled around I would get panic attacks and was unable to tolerate it. Why?

Ritalin (methylphenidate) Hormonal changes likely. Consult your ob/gyn.
Anxiety. Stimulants are anxiogenic (anxiety causers) in some people. When period comes, you are thrown into neural imbalance. The excess Dopamine from stimulant down regulates serotonin production in your brain more so at these times, & anxiety irritability or panic occurs. Try to avoid caffeine & other stimulants like chocolate during your period. Check with your doctor for management of this problem.
It intensifies mood. Ritalin (methylphenidate) and similar meds can "amplify" emotions as well as heighten focus. It's important that mood - including pms-related mood changes - be appropriately treated when these meds used.
See your doctor. Many people with adhd also have panic disorder. Panic symptoms can worsen around the time of your period. If you just have peri-menstrual symptoms, your doctor may be able to give you medication to help you cope but if you have panic or anxiety symptoms more than 1-2 times/week, you probably need daily medication.