IP address and speed

For months my IP address has been the same. The suddenly my speeds went through the floor for about a week and my IP address changed. I now have an excellent connection and my IP address has gone back to as before. I have looked at the Speedtest logs and while my speed was slow I had a different IP address up to the very hour that everything reverted to normal. Why would this happen?
I'm referring to my Virgin Cable line (see below), not to be confused with BT whom I know change IP addresses frequently.

Without wanting cause offense is it possible you was connected to a neighbours wireless connection or similar? (just because you mention you had trouble with wireless)

Perhaps to rule that possibility out, enter the IP that you had into a "whois" and find out if belongs to Virgin.

Of course perfectly possible virgin were doing something funky...

VM may have moved you off your previous router and onto another one while they replaced line cards in the previous one. Now that work's done back to the original you go.