My question is simple: what if a Muslim says, "Jesus is my God"? What will happen? What if the person who says that was a Christian before? What if it is a female and she was emotional and stupid?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Any Muslim who says these words has committed Kufr (disbelief) and is declared an apostate, whether he was a Christian before or not, and whether it was a man or a woman. As long as the speaker said these words willfully and he was not forced to say them, he is considered a Kaafir (disbeliever) and an apostate; may Allah guard us. He must repent to Allah, declare the two testimonies of faith (take the Shahaadah), and bear witness that 'Eesa (Jesus) is the slave of Allah and His Messenger and His Word which He directed to Maryam (Mary), and a soul [created at a command] from Him. If he fails to do so, he is subject to the wrath and punishment of Allah in this worldly life and the Hereafter. If he dies in such a state (before repentance), he shall abide in Hellfire eternally. We ask Allah to safeguard us against such a fate.

Allah knows best.