Hello! A strong possibility might be a tick. Emma Norris wrote a good blog about ticks in March of this year (see
our Vet Help Direct blog) and although the picture in that article looks nothing like your photo, ticks can, once they're attached to the skin, look similar to your dogs' temporary lump.
I'm not sure which country you're in regarding tick-transmited diseases, but you might want to speak to your vet in case there's anything special that they'd like you to do now. Tick preventative treatment may certainly be advisable going forward.
You should also check the rest of your dog and your own body for ticks because they grab on to people too.
All the best and please let us know how you get on!