Here's what happens:
* Natsu's fight against the Phantom Lord Guild: He is fighting against the powerful Phantom Lord Guild members and things are not going well.
* His opponent, Jose, uses a powerful magic: Jose, a member of Phantom Lord, uses a magic that weakens Natsu and causes him to lose his strength.
* Natsu's frustration and determination: Seeing his own power diminished and failing to protect his comrades, Natsu is filled with frustration and despair. This is why he looks like he's about to cry, but he doesn't actually shed tears.
* His determination to overcome: However, the sadness doesn't last long. Natsu's fierce determination kicks in, fueled by the need to protect his friends. He regains his fighting spirit and continues to fight.
Therefore, while Natsu doesn't cry in episode 99, he does experience a powerful wave of emotions, especially sadness and frustration, which is reflected in his facial expression.