Can Masturbation Be Performed in Any Way

I just wanted to know if there is a limit to how many times women should masturbate in a day? I've found I can go about 5 times with only a break of a minute or two in between. Is this likely to cause harm? Also I am wondering if it is 'normal' to masturbate with clothes or undies on (this is how I prefer it). I usually do it lying on my stomach rubbing the vagina with the palm of my hand (from behind). I try and sqeeze my vagina after a while so I can feel it contract as this feels good to me. This is especially helpful when I have my period because I don't want to make a mess. However I am concerned it might not be healthy to masturbate at all when you have your period?