How To Calculate The Ovulation Date?

Hi , I am 43 years old , I got my last period on the 7th of December , I ovulated on the 17th of the same month , I know cause my temperature was above 99 degree , my breast got hard and become hurting , I felt like I got pregnant so I check my temperature every night , it was sometimes 98.3 sometimes 97.4 never below , I got some spots but not a big deal and some cramps , but today i have much more blood just like my usaul period is this an abortion or I am going on pre=menopose ? I want also know when will be my next ovulation cause here I am confused how to calculate it , cause i want really to fell pregnant , thanks a lot for your help