Line on OPK 5 days before AF

Very very confused at the minute.

Period is due this Sunday (18th).

Got a positive line on the opk on the 3rd. Negative (no line at all) on the 4th.

Just took the opk now and gotten a light line in the test window.

Period is due in 5 days though. What on earth does the line mean? I shouldn't be ovulating until next month now.