The verb to creep is traditionally inflected crept in the past tense and as a past participle. Creeped has crept into the language, however, and is now an accepted variant, though it might be considered informal or dialectal.
Crept is still considered preferable to creeped in almost all cases, with one main exception—in the past tense of the phrasal verb creep out, meaning to strike [someone] as weird in a frightening or off-putting way. As an inflection of this phrasal verb, creeped out is more common than crept out. Your spell check might catch creeped even when it’s used in creeped out, but spell check is wrong in this case.
Crept, as used below, is the more common past tense and participle of creep:
In Europe, most markets crept lower amid worries about Greece. [Wall Street Journal]
The mobility scooter is an institution that, like most others, has crept up on us. [Independent]
But when she went to college, the pounds crept back. [Hamilton Spectator]
And while crept is usually preferred over creeped, the latter works in the phrase creeped out—for example:
I found myself creeped out to an extent that no horror story had achieved for a long time. [Guardian]
I’m fairly sure I creeped some people out. [Toronto Star]
For his part, Dan is just creeped out that she went through his stuff. [Los Angeles Times]