Definitely forgo the new lingerie – save them for another time when you’re not risking destroying them. It’s not worth it.
Do you use a condom when you have sex? (I hope you do!) If you do, then go right ahead with the plans for today. Getting cramps doesn’t mean it’s going to happen tonight and if it does, the worst thing that could happen is that you’d have to stop.
Is it you or your boyfriend that would be opposed to have sex while on your period? Might be a good time to talk about it?