The Dungeon Master's Guide has two recommendations for DMs when a new player joins an in-progress campaign. The first is to have the new player create a character at the same level as that of the lowest-level existing character. The second is to have a player who is brand new to D&D create a 1st-level character.
(See also: At what level should you start players joining an ongoing game? in which a player who's completely new to D&D (3.5e) is joining a group that are 9th level.)
However, both of those are recommendations, not rules, and they are given because it's easier for the DM when everyone is at about the same level. But there's definitely no requirement for level parity, and in fact, the disparity can potentially be used to create some interesting dynamics that might never be seen in a perfectly-balanced party.
Don't worry about the fact that your PC is a lower level than the other players'. Part of the DM's job is to make sure everyone is able to enjoy the campaign. If he finds himself having trouble building interesting encounters that are compatible with a 2-level variance among the PCs, then it's up to him to solve that problem, perhaps by creating a few custom-tailored microchallenges for you so that you can earn a bit of extra xp to catch up.
The only exception is when the level gap is very large (more than 4 levels), at which point any challenge is either too easy for the highest-level characters, too deadly for the lowest-level characters, or both.
 Dungeon Master's Guide, pg. 236