You do need about 3 seasons to see a pattern. Sometimes second one can take a while to come in but third and subsequent may settle between 6 and 8 mths in regularity.
TBH sounds like it may be just around the corner, may even have been silent and you've missed it especially if she's an only dog and doesn't get the opportunity to mix with other ones, especially those all knowing male ones.
she is an only dog but we meet every day with lots of other dogs.....no sign of it happening.....do you think i should take her to vet? can they check? we want to get her spayed after 2nd season...thanks
Personally I wouldn't bother, if she's had one season she's going to have another (from my experience anyway). Its just a waiting game, once she's having her season you need to book her in 3 mths later at the vets.