At my last check up, I forgot to tell the doctor about asthma. Is that ok?

Remind the doctor. It would be helpful if your doctor knew about your asthma. You may need medications for it and your physician should know which medications you are on. I suggest you contact the doctor's office and let them know about your asthma.
No!!! 5, 000 deaths/year and 500, 000 hospitalizations for asthma attacks per year. Even just call and update the office with this information. And you need to see an asthma specialist to ensure you are fully controlled. Don't wait until something bad happens.
Update doctor... It is ok if you forgot to tell your doctor about asthma to let your doctor know at your next appointment. If you don't have a follow-up appointment, be sure to make an appointment before your asthma meds run out to discuss your asthma symptoms, how well-controlled your asthma is, etc because your doc may want to do some testing first to insure you are on the best regimen possible at this time.