Late or missing heat cycle

I have a 3 year old bully who has not had her heat cycle since January. I have read that a bulldog may go anywhere from 4 - 10 months between heat cycles.  She was pretty regular every six months in the past. Any experience with this?

Thanks

Regular?

What was regular for your girl?  I have experience with cycles 13 and 18 months between....and they were healthy and normal, no thyroid issues.  However, this is how they were to start...not shorter cycles that got longer....and the 18 month cycler just had a heat early -- 10 months.  There is a wide range of normal in Bulldogs.