I received this message:「行けることになりやした」. Doing a quick Google search for なりやした, I found a few phrases like 「新米母になりやした！」and 「遅くなりやした。」
Is it a kind of slang for なりました?
It's used by characters in movies etc. depicting typically edo-period. It's allegedly how lower-ranking (lower social status) people talked to higher ranking people in those times. I don't know if this is actually the case, but that's the stereotype.
When used in today's setting, the speaker is joking.