I'm going to stick up for your mum a little. :-)
Yes, you had a great day, and I'm really pleased that your boyfriend came to keep you company, as a mum I would be happier that you had company.
But it is her home and her (or your dad's) things, she obviously does not know your boyfriend very well with him being new and does not know if he will take care of her things or not and I guess does not put trust in basically a stranger to her.
Some people really don't bother about such things but if she is finickerty about her possesssions then she will continue to be so no matter how old you are, which you should have figured out, there may also be some resentment of the new boyfriend in there too, infact I would think there is, I get the impression that perhaps for whatever reasons she did not want him there and begrudged him spending the time with you, which is why she begrudged him using her things. I guess a more indepth talk as to what she thinks of him may be needed.
Sorry, your day was spoiled, it's awful when you've had fun to be brought right back down to earth again. Just make sure the bike is clean and next time, just don't tell her. :-) Or don't use her things.
Oh dear, bet your glad you are living back at home.