Mid-cycle bleeding that follows a pattern

[elettaria]

I've been charting for a year now, and noticed that my cycles form rather an odd pattern. I get lots of eggwhite, often all the way through my cycle, which I've been told here is normal with a copper IUD. I also get mid-cycle spotting (though today, CD15 which is far too early for a period, it was more like light bleeding - it it had occurred a week or so later I'd have thought it was my period). Now, IUDs can cause some spotting, but as far as I know the spotting caused should be random, and mine follows a set pattern. I get about a week of spotting mid-cycle, corresponding almost exactly to the time when women without IUDs would be getting fertile CF, and stopping at ovulation or up to a couple of days past it. I also have a short luteal phase, which I'm trying to treat with herbal medication and nutritional supplements (getting up from 7-8 days to 9-10 days so far). Any idea what this type of spotting could be a sign of? I can't see any reason why an IUD could cause spotting only during the fertile phase like that, so I'm assuming it's my body causing it. I know that a day or so of ovulatory spotting is caused by a drop in oestrogen, but what's going on when it lasts a week?

ETA: We're now having an interesting conversation about a type of computerised fertility monitor called LadyComp, if anyone's interested.