OK I googled and found this
Does having an LH surge mean that I will definitely ovulate that month?
Not necessarily. In a normal cycle, ovulation usually follows the LH surge. But sometimes you can have an LH surge early in your cycle, before the follicle is ready. If this happens, you’ll have another LH surge later on in your cycle, and ovulate after that one.
Another reason why ovulation may not follow the LH surge are certain conditions that cause elevated LH levels such as PCOS.
And finally, a third reason why the LH surge doesn’t always lead to ovulation is a phenomenon known as luteinized unruptured follicle (LUF). Between 6 – 25 percent of women with infertility have LUF[2.Qublan et al. 2006], and it’s one possible cause of unexplained infertility. It can also occasionally happen to women with otherwise normal cycles[3.Killick et al. 1987].
In LUF, even though hormone levels leading up to ovulation are normal, the follicle doesn’t rupture, so the egg is never released. In normal menstrual cycles, the ruptured follicle becomes a corpus luteum, which releases progesterone and stimulates the lining of the uterus to build up. But with LUF, even though the follicle never ruptures, it still releases progesterone. From the outside, your cycle will look completely normal: you’ll still get a regular period each month, and you’ll still get positive results with the temperature method or ovulation sticks. In fact, the only way to detect LUF is via ultrasound.
Well that would explain my random positive opk so early.