hello,
if shes ever been close to a male she will most likely be pregnant,also if you can see her gravid spot ( black spot ) close to her anal fin i would say she is, mine gave birth roughly every six weeks, ive never heard of it going brown though, some fish dont get really fat some do, when she hides a lot thats one sign shes ready to give birth, as for the male guppy they will mate with any thing that swims lol, right now my male is trying it on wihh a platy

, hth all the best with the fry, donna

.