Blimey, that kind of thing would have had me completely frozen. The slow build over three nights must've been unbearable, just lying there each night convincing yourself it was pipes or the house settling.
What did you actually find when you looked? That's the part you've left us hanging on mate. Under the bed is one of those spots that feels so exposed when you're crouching down with a torch at 2am, I've done it once after hearing something similar in my old house in Leamington and honestly the dread of looking was worse than anything I actually found (turned out to be a mouse, but still).
The breathing detail is what gets me though. Scratching or movement you can explain away fairly easy but breathing has a very specific quality to it, its harder to misidentify. Did it sound like it had a rhythm, like something genuinely alive, or more irregular? Really curious what your conclusion was on this one.