I saw the video. Ok, i believe there is a male guppy chasing her in the 20 second mark. (there appears to be a gonopodium on a blackish guppy i believe)He's onl chasing her for 5 seconds or so. If she is heavily pregnant, he would normally be chasing her like the boxing day sales bargain hunter for a lot longer. However, she has got a pretty big belly. Did this arrise over night? If not, them possibly pregnant. If so, then stop overfeeding, or she could be constipated. I read somewhere that feeding cooked crushed peas cures that, but if it is the case, then asking "wilder" in the tropical fish emergencies section might be necessary.
If i had to make a choice, i would go with pregnant. I would prefer a more skilled oppinion though.