At the start of issue 171, Siddiq claims that Rosita was in love with him and that she would dump Eugene to be together. In Issue 133 and Eugene's flashback, Rosita told Eugene she never loved Siddiq and vise versa. Was Rosita only saying that because Eugene found out or was Siddiq obsessed over Rosita and made himself believe their relationship was more than what it actually was.
Rosita told Eugene that there was no love between her and the guy she slept with, Assuming that Siddiq is the lover and no one else, I think Rosita lied because Siddiq claimed they loved each other and that Rosita would dump Eugene for Siddiq.
Yes, Rosita was lying. She loved Siddiq more than life itself but she couldn't bring herself to betray her best friend any longer. There is so much evidence of it, I put it all up on The Walking Dead Comics and NSFW Walking Dead Comics (18+) on FaceBook, I just cross posted it. To summarize, Andrea was the first to know that Rosita was carrying Siddiq's child. She would never tell anyone she knew, because their bond is deeper than sisterhood, even after death. She may have told Eugene first mostly out of being unsure what to do, and asking Andrea (a very moral and loyal person) for advice and thusly waiting before telling either of them.
So timeline. Issue 127, Andrea bugs Rosita about telling Eugene. She makes some excuse.
Issue 128. "Out late tonight. Don't wait up. -R" 100% with Siddiq. The importance of this is that she continues FUCKING Siddiq even after she knows for a fact she's pregnant. Either that's some insanely good dick or she is seriously considering staying with him.
Then the confession in 133. She says she never told Siddiq, but she didn't say she didn't want to, rather, Eugene demanded she doesn't, and she complacently agreed out of guilt. The guilt continued all the way to the fair, where she ran away from Eugene and was captured by Alpha. At that point it's anyone's game--we don't know for sure if she was running to find Siddiq, but I have my suspicions.
Now, Siddiq is a bit derpy, but he isn't delusional enough to invent that Rosita was going to leave Eugene. She said it at some point, either before or after she knew she was pregnant. My thought is after. She didn't tell him she was pregnant, but she said "I have to leave Eugene" without specifying why. Then changed her mind out of guilt.