If your piggy jumps, or makes a kicking motion while petting her, then she's annoyed. Mine will sometimes toss their head up if I'm petting them too much. There are several different types of purring. A lot of what it means depends on the pitch. Listen closely to your piggie and after some time, you'll recognize them easily. Rumblestrutting is for dominance or mating. This can happen between males and males, males and females or females and females. Rumblestrutting is a long, low pitched purr that is normally accompanied by butt-dragging, chasing other piggies, and mounting. All of these are signs of dominance and can happen during a mating call or while sows are in heat.
There's a contented "lap time purr". The piggie usually lies down contentedly (I call it melting into a fur puddle). The purrs are short, and sometimes can't be heard-you'll just feel the piggie vibrate or shake on you. The "annoyed" purr can also happen during lap time, I've had it happen if I want to pet one of my girls in the cage and they're lying down. They're basically telling me "not right now, just let me sleep". It's similar to lap time purr, but I find that sometimes it's slightly higher pitched.
Like I said, you'll learn to tell the difference in time. Hope I've helped you out a bit.