Without seeing her, I'd suspect a severe intramammary infection (mastitis) that is indirectly or directly related to her not completely cleaning after calving. Often times problems like that come together. Most of the time a cow with mastitis will only have a local infection (within the quarter), but sometimes it will go systemic (an infection throughout the body), especially if you're dealing with ecoli.
That's my thoughts. I've had more success with long-lasting pennicillin than LA200 when it comes to ecoli. (For that matter, I've had more success with penn than oxytet when it comes to any type of mastitis.) Often times goes by the brand name "Twin Penn"; I think it's from AgriLabs. I'd use 60cc's, twice 24 hours apart, then go 48 hours and retreat again.
Strip that quarter and see what color the fluid is in it - if it's clear, off white, hot, or watery with flecks, it's very probably ecoli and I've seen it kill cows, directly or indirectly.
Did you check her temperature?
Best of luck with her.