Reacting to other peoples' pregnancies.

I have found over the years that I have reacted very differently to other people becoming pregnant.  I reacted very badly to both my SIL.  I feel numb towards my sister and really want to keep my distance. It is early days for her but I wonder how my feelings will progress.  My colleague has just been rushed off on maternity leave and actually I am getting into it by suggesting a baby mock up of a company t-shirt and I gave generously to her baby present fund!!!!!!  Previously, I have been in tears at my colleagues getting pregnant.  A couple of receptionists at my other job have toddlers and it is baby talk all the way and I am up and down.  I think because I spend so much time with kids as a teacher, I am able to get involved with their conversations so I feel included, so maybe working with kids has helped me.  I even call my classes 'my kids'. I don't get jealous at all.  I have had a couple of sticky moments with parents and I feel quite useless when that happens. Perhaps my job gives me a child fix???  Could this be that I am starting to deal with my infertility issues?