Hi Ellie. The length of your LP depends on when you are ovulating. You need at least 12 days to have a chance of sucessful implantation. Also, you may not be ovulating at the time you think you are (unless you know how to spot the signs) and could be timing it at the wrong time in your cycle.
I had a reasonably regualr 26 - 28 day cycle throughout my time ttc, but sometimes wasn't ovulating until as late as day 18 or 20 which was leaving me with a woefully short LP.
If you do have a short LP, there are things you can do to help boost it. eg agnus castus, progesterone cream. I tried these and they made not a jot of difference, but as always, everyone is differnt.
I wouldn't have thought that the increased feeding would have caused your miscarriage, however the level of feeding beforehand could have meant that the conditions weren't right to sustain a pregnancy more than a few days beyond conception.
You will find that MN is full of people who have sucessfully managed to conceive whilst breastfeeding, and I know I felt like I was the only one who couldn't, but here I am 12 weeks pg and ds is still feeding morning/bedtime and sometimes during the day too.
I guess you have to figure out how far you have to cut down (if that's the route you want to take?), or whether you are willing just to wait it out until your ds cuts down himself.
Let us know how you get on!