You do not have to go back to the meeqaat, rather you should enter ihraam from the place where you are when you decide to do Hajj, whether that is in Jeddah or elsewhere. This is because when you passed the meeqaat you were not firmly resolved that you were going to do Hajj.
Shaykh Ibn Baaz said:
Whoever comes to Makkah with the intention of doing Hajj if he can, then it becomes possible for him and he decides to do Hajj, should enter ihraam from the place where he is, whether he is within the meeqaat or in Makkah. If he knows that he will be allowed to do that, then he has to enter ihraam for Hajj from the meeqaat which he passes, when he passes it with the firm resolve to do Hajj. The Prophet (peace and blessings of Allaah be upon him) said: “They [the meeqaats] are for them [their people] and for anyone who comes to them who is not of their people and intends to do Hajj or ‘Umrah. And whoever is living within these boundaries can enter Ihraam from the place where he decides (to do Hajj), and the people of Makkah can enter ihraam from Makkah.” Agreed upon.
Fataawa Ibn Baaz, 17/53.