Why wont my Siamese cat stop sticking out her bottom?

It seems as though she is in heat but yesterday a friend came over with a dog and she hissed wildly and ever since she has been much more friendly to all of us and has been rubbing herself all over maybe to spread her stench around to limit her territory but all I know now is that she is much more lazy and wont stop purring or sticking out her bottom when she is resting. the movement I mentioned is shown in the picture above. The cat is 2 months old and her name is sushi.