you seem pretty sure she cheated on you...
here is what i would think if i was in your position,
1. She is tightening her pussy so she can feel more,
(most girls will lie about doing this)
2.She used a tightening cream for you to make you feel more.
3.She maybe was not "fully aroused" or "tented" and
very very wet...hence you could get in,but she was tight
and wet rather than soaked and gaping.
try more oral is you like more space,
get in her head,kiss her and look at her whilst you kiss,
breathe freely whilst kissing,do not hold your breath,
just envelope in her everything and let her know you
want her to loosen up,you like to fuck "open" pussy.
just some ideas.:redface:
i doubt she cheated on you,maybe she just now truly realised
that you have size?