Mama2PudgyBunns...if your cycles are all over the place, I think that charting would be a good idea. Chances are, you are ovulating at different times, so it would be easy to miss.
My cycles were all over the place after dd2 (got AF back at 13 months post-partum and I'm tandeming nursing my 2 daughters). Really weird stuff...some 6-7 week cycles, some 3 week cycles. Even had a month of constant bleeding (checked it all out - it was to do with breastfeeding hormones).
Anyway, I'm now 20 weeks pregnant...about a week further along that I'd originally thought, as I'd ovulated early (for me). I always have 5 week cycles (when I'm regular), and my cycles were slowly moving back towards a 5 week pattern...so I just assumed I'd ovulated around day 21.
But at my scan, I was a week further along than I thought, so I must have ovulated early.
All of that to say - with irregular cycles, you could well be ovulating at odd times, so charting might help.
Easier probably than trying to have sex every other night 'just in case'!