Rules question :)

Player A plays a shot and leaves a red ball sitting on the edge of the top pocket and is covered and player A walks away. Then player B (me) is snookered. I take my time to work out how to get out of the snooker. I play off 2 cushions. The cue ball is heading towards the red - looking real good - cue ball is just about to hit red but the red ball falls into pocket just before cue ball hits it and the cue ball follows it in.

So what happens? Is it just bad luck and costs a 4 point foul.

Or is it replaced and the cue ball replaced too?

Rules question :)