My Sister Is Mad At Me Because I Befriend My Ex Boyfriend?

Okay so me and my boyfriend whom i in love with just broke up a week ago because he said he wasn't ready for a relationship and because hes young and still wants to hang with his boys in which we actually stayed together. it hurt me but i was kind of okay with it because i feel like we jumped in it to quick we were only together for 5 months and went through more problems then a married couple and i have a 5 month old i dont need the stress. But after we broke up my sister Whom Is 3 MONTHS PREGNANT was basically telling me to not talk to him and to avoid him for a while so that he can miss me because for one he would always choose his friends over me. But i simply told her that she was right but to be honest i dont even want to be back in a relationship with him hes not on my level. but me and him talked the next day and agreed that we should be close friends. the day after that i asked him to chill with me because i was bored was off of work and everyone was chilling with someone but me. he told me yeah but he ended up getting wasted and going to sleep. the next day he explained it to me in which my sisters boyfriend is his bestfriend and he tells me everything. but my ex asked if i wanted to chill i said yeah. i ended up spending the night and when i came back my sister went crazy on me saying im being dumb. the next day we chilled again and when i came back the next day she seen a hickey on my kneck . yes we did have sex but it was because i wanted to NOT HIM! And she went crazy again saying "WHY DID HE GIVE YOU A HICKEY YOU GUYS AREN'T TOGETHER" and i didn't understand what having a hickey has to do with being together. Then Day 3 Of Us Chilling which was last night she texted me out of no where saying that im being dumb and that he dosent care about me nor does he do anything for me and that shes done with me and that she wants me out. i had no problem with leaving because i was only there to be closer to my job and because SHE ASKED ME TO COME. she swears i came to be closer to him in which i couldve went home and still came over so thats not even the case. but i told her that she was wrong not about him but about her saying shes done with me because i want to hang with my ex. which i feel is my decision even if shes right she cant hold my hand all of my life. but she really got pissed and started texting me saying mean stuff and it kind of hurt but for some reason i feel like either someones talking to her or shes mad at something else and taking her anger out on me because minutes before i left we was okay i even told her i was leaving to his house and she was okay with it.but now that shes pregnant everything ticks her off. especially her boyfriend when he leaves to go chill with his friends but they be together 24/7. But really i need advice. i clearly understand and i tell her all the time that shes right but i feel like shes taking it to the extreme just because im chilling and being friends with my ex. i love my sister but right now i dont know what to do and to be honest i told her which is true that the only way i get over guys is to be friends with them because eventually i will fade away from them. and plus i sleep on a leather couch at her boyfriends house and i mean me sleeping in a bed was a gift to me. lol seriously. and then she went crazy about my son because for one i begged my sister for months to watch my son for me while i work she says no i told her i would pay her and she tells me about day care. i dont trust my son in day care at this age. i wouldnt mind but i cant do it. so let my god parents her him for a week and she went crazy on me as if i was giving my son up or something telling me i need to go get my son. my son was coming back this weekend idk why she even said that. but advice please on maybe why shes acting this way