20 day-ish cycle after mirena in for 6 yrs

Richelle
I've been trying to get pregnant for 7 months with no luck. I had mirena inserted for 6 years before it came out in the summer. 
​I am almost 36 years old and have 3 girls (6,10,13). 
​I've taken ovulation kits and have charted my temp just the past month. Last month there wasn't a rise in my temp. 
​My period has consistently been very short, averaging 19-21 days. It's been hard to pin point when I ovulate  
​We are very active sexually, averaging 4-5 times a week. 
​Any one else have experience with such short cycles and getting pregnant?