Off depo for 18months thinking im pregnant now?

i have been off depo for 18months.my periods are normal and they last 5 days.i went on this site to get the date of my ovulation date which is the 18th of this month well me and my fiance had sex on the 15th 16th 17th and 18th..and last week i spotted one day one time in the morning then stopped this week i spotted one day one time..i have been feeling tired,dizzy,and having excessive headaches that do not want to let up..i don't have insurance so doc visits are too high just looking for an answer that will help..thank you