so confused

so i found out at the beginning of volume 3 that:

(volume 2 spoilers)

the guy iwaya dated after the guy who cheated on him was just a random guy. i really thought it was hasumi. so they had sex at one point after that dinner. then on the next chapters after that gossip about iwaya being pregnant after she suddenly stopped smoking, on her own thought bubble, she claimed she never did it with a guy? then what was it she did with the 4th guy?

no they didnt? the only person she ever had sex with is her sempai.

this one:

v03c01 p05 reference to guy #4
http://i.imgur.com/9U0Up.png

sex with guy #4
v01c01 p36-p38
http://i.imgur.com/igf08.png
http://i.imgur.com/flBB8.png
http://i.imgur.com/olqD0.png

hmm, you are right! holy cow, i lack reading comprehension. probably cos the face of 4th and 5th guy look almost the same.

i feel ya man. i was confused for 2 volumes thinking he was hasumi but he's actually another guy after i read that part in volume 3

well anyway, its not like you always get pregnant after sex. or 4th guy probably had rubber on

I don't see any continuity errors. Here's the timeline as I see it:

Prior to series start:
*Loses virginity with Hasumi in school
*Gets dumped by Yoshida (3rd guy)

Chapter 1
*One night stand with 4th guy

Chapter 2 and on
*Starts seeing Hasumi

Chapter 6
*Girls gossip about Iwaya being pregnant. Since it's known in the office that she's with Hasumi, they assume it's his.
*Iwaya thinks, "We haven't even had sex." This is true. In the current incarnation of their relationship, they had not had sex yet.

Chapter 16
*Now Hasumi and Iwaya have sex.

Lol, you're confused 'cause you're confusing the characters.
She did it with the 4th guy, 'nothing happened' could just mean their relationship did not work out as expected.
Started dating the 5th guy (they become a famous couple that everyone knows about) and that's when people started gossiping about her being pregnant because she stopped smoking but she didn't have sex with him yet.
;P Remember the names, it'll help.