Why do girls move on so quickly?

ok me and this girl been datin for about 5 or 6 months..and now were done. we were very serious..talked about engadment and evrything. But the maint problem was we argued alot. and she comes to me and tells me she needs time (a break) so in that month we were on a break somethin seem kinda fishy..she was very short with her words(texting) and evrything..like ill ask how shes doin she say "im doin fine" so finally i asked wat the deal was like is she talkin to anyone and she come out and says yea kinda..but hes a family friend and we only hung out? do you guys beleive that crap?...so all i wanna kno is why do u think she moved on so quickly? we seemed like the the perfect couple..i mean how coulkd sumeone start talkin to another guy so quick? im really upset...is there a chance she may come back and she just doin this for attention or something? i really need quality answers please!