Is there any way I can confirm, that someone else generated a block for the same block as me, and then it forked, and the other chain happened to grow longer faster?
Search debug.log for "REORGANIZE" right after your "proof-of-work found". That means another chain has gained the lead over your chain.
That's why we have this...
// Don't show generated coin until confirmed by at least one block after it
// so we don't get the user's hopes up until it looks like it's probably accepted.