I would go for it and stop your periods for awhile if they are affecting your POTS. I had put my period off for 3 months and was feeling fab and then a month ago I had a period and the doctors think it has bought my symptoms on. Periods are a big trigger factor for POTS apparently. The oestrogen in the birth control pill is supposed to expand blood volume which is good for women with POTS. I appreciate you are looking at an IUD as opposed to the pill however I think it's worth stopping your periods for a bit and maybe your other symptoms will lessen somewhat?