How to know if a guy is bored with you?

i've been in a friends with benefits relationship for the past month now. we kept it on the DL the whole time so no one would suspect at work (we both work together) and everything outside of work was pretty nice. we had sex a few times, but it wasn't that great, but the foreplay was. in the beginning he would flirt with me A LOT, and me being the natural flirt, would do it back and that's what started our relationship. Everything was fine, even just two days ago, we had sex again and we hung at his place after we slept together for a brief period and just talked.... about him -.- at one point i thought he was developing feelings for me, as he would try to show me all of his little accomplishments and try to get me to know him better and **** which put me off a lot (which was part of the reason why i wanted to break it off) he was still interested until tonight when he got into another female coworkers car! she took him home! lest i remind you, i was still "meh" about the sex and his personality in general, i just found him physically attractive, and i'm just a territorial person by nature (i'm a scorpio) and was floored! i was going to dump him since the sex wasn't good, but i think i just got played/dumped! what's worse is tgat girl who he went home with and TOLD me that he told her what we'd been doing, but she claims that she didn't do anything with him (lie). so that constitutes as moving on for him? i'm just shocked more than anything. i'm not hurt though, even though i'm pretty sure our fwb thing is over. WTF? any advice would help greatly.