Red Small Colour On Tail

[zain611]

I noticed on my black male guppy has 2 small blops of red colour on the edge on its tail. it looks like a bit of red paint went on it. what i think may of caused this is that i have small bits of gravel at the front of my tank which my cories dug up from the sand so i was thinking a small red gravel went on its tail but theres 2 blops of red not 1

 

[fluttermoth]

Gravel wouldn't stick to a fish like that.
 
How old is the guppy? They do develop different colours as they mature, so it could be that. Do the red spots look swollen or fluffy at all?
 
Have you tested the water (if you have, could you post the actual numbers, please?) and done a big water change, to be on the safe side?