Many things can change your cycle. Stress being a major one. Also lack of sleep, lack of normal sleep patter, diet, lack of exercise....the list goes on.
They do sell ovulation tests, though i'm not sure how they work. They do have calendars and I think you just plug in when you had your last one.
Hopefully the future father is in on this. If so, just have lots of sex. Sperm can stay inside you for up to 3 days. Eat healthier, cut out junk food, sodas, excess sugar, fat, oils. Exercise more, go for 1-3 mile walks around the neighborhood 3-5 times a week. Cut down/elimenate caffine. Gett at least 7 hours of sleep a night, go to sleep around the same time every night. Take prenatal vitamins.
LOTS of sex. Even when you don't necessarily feel really worked up. When he cums, have him stay inside you and deeper. The closer the guys get to the cervix, the easier time they'll have getting through. The vagina secretions (that get you wet) are acidic and meant to kill infectious microogranisms. This will damage/kill sperm too. After he pulls out, put a pillow under your hips and allow gravity to help. Stay this way for a bit and then go at it again. The more baby making stuff you have the better the chances his guys will have to get the sweet spot. Hopefully with timing and all poof, you get pregnant. Good luck!