Ex girlfriend dating my friend a month after breaking up. Pls HELP!!!!!

Month after we break up my EX is dating my friend. Pls Advice!!! This might read a bit long but will take less than 2 minutes.

I am a 28 yrs old guy and I met a girl (26 yrs old) 10 months back through some common friends. She was already in a relationship for last 3 years and had plans of getting married to him. Over the next few weeks we got close to each other and she told me about her bf issues, while she was sorting her issues with him, we ended up having sex one night. Few days later we realised it was wrong and she should either break up with her bf or we should stop our intimacy as we started to like each other. She broke up with him a week later but wasnt sure if wanted to get into a relationship with me as it was too soon for her to start a new relationship. We kept being intimate for couple of weeks till she took a month off to travel alone. While on her travels she met some guy for a week and ended up sleeping with him which she told me a week later after she returned. We had a big fight over this but I accepted her back in my life knowing that i really liked her and technically we were not in a relationship when she slept with this random guy (She argued with me over this point) although she did lead me on by calling me every day while she was away.

Over the next few months, things got better and we got more and more into each other. There were somedays when she would give me a cold shoulder and tell me it was coz she would be missing her ex bf a lot and wasnt feeling it for me, which i kind of understood. While we were getting serious over the past few months, she started making plans for marriage and convincing me of our future, since i really liked her i convinced myself of thinking long term untill a month back she felt cold again towards me which made me realise its better to put this relationship off than to drag it. She wanted some more time even after 9 months to understand her feelings about us which i wasnt ready to give and i told her we should call it off and not be in touch for a while so that she can think over it and understand if she wanted us together or not, and so we took a break.

Its barely been a month since we broke up/took a break, and i found out through common friends that last week she started to date an old friend of mine who didnt know about our relationship. I dunno what to do and i am so mind fu*&#@ed. I thought i was moving on well without her until now, that she has started dating a friend of mine and was getting cosy with him at a friend’s party. Please somebody advice how to deal with this..? I always had trust issues with her coz of her promiscuous past, and now its turning into a reality.

Should I confront her or let her be? Should i take her off my Facebook and have no contact with her or act all cool and casual and not let her know anything. If i give her attention she will be pleased about it, learnt from her last break up with her bf. Please help!!! I end up going on her facebook page everyday to check on her and I do not like this.