While taking accutane, my period was extremely light and probably lasted for two days or so. Now, 30 days after finishing my last cycle of accutane, I have started my period and it was heavy for the first few days and has continually gotten lighter; however, it is lasting longer than my usual period (going on 9 days now). I'm on the pill this week and not the placebo pill so I shouldn't be on my period at the moment. Additionally, my boyfriend and I had sex about day 4 of my period (it was very light that day and I , so I'm not sure if it's just a slight tear from the sex or just because my body is adjusting to not being on accutane anymore. Before this turns into a lecture about sex we are safe, I am on the pill and also use condoms.
Yes this can be common, in fact Roche has stated that one of the side effects is abnormal menses, what they don't admit to is infertility and sexual dysfunction. Looking at it abnormal menses can mean a lot of things. There is thread on another forum regarding infertility.
I would suggest first that you get some labs and see a doctor and they also state that Accutane works up to 3-6 months after stopping so maybe see how next month goes.
Good Luck!