This sounds to me like it could be a spirit. Spirits often appear as simple balls of light since that is, you could say, more like their "true form." A physical appearance is pointless outside the material plane, but is often used by spirits so the living can relate easier. My guess, just at a glance of the details, would be that the spirit was an old friend, relative, or possibly although less likely a pet that your mother had for awhile just passing through to say Happy Birthday.
That would not really be as uncommon as some might think. Spirits have also been known to return to their grave site upon the anniversary of their death for a variety of reasons.