Need help

To start I was on birth control for 5 years, Mirena IUD. My fiance and I decided we wanted to try and get pregnant again, we already have 2 children. I had my birth control taken out about 8-9 months ago. My family doctor assured me my periods (which stopped while on bc) would begin within a few weeks and I would start ovulating within weeks. Sure enough my period came about 3 weeks after being off birth control and we have been trying to get pregnant. My concern is my periods have been having shorter and shorter times in between. The last 3 months I've been having my period every 14-18 days but they have only been lasting 3-4 days but very heavy the first day or two. I'm not pregnant obviously or I would be ecstatic! Now my question is with my periods coming so soon am I even ovulating? And mainly is this something whether I am or not I should be concerned about? I've been trying to wait it out to get pregnant which we are very much trying or to see if atleast my periods will go normal. But I'm just getting very frustrated.