Following a miscarriage it is very common for a woman's cycle to be irregular or change from its previous pattern for a while. Often it will return to normal but this may take several (meaning >6 in some cases) months.
Regarding your question about ovulation, ovulation is generally thought to occur 14 days before the next period is expected. With a 28 day cycle this is mid-cycle. With a 24 day cycle, ovulation would be expected on day 10, slightly earlier than mid-cycle.