Any problems you have will be 100% mental (disappointment at not being able to go vegetarian, taste/texture aversions, etc). It takes a lot longer than a month for your body to adjust enough that not having something would cause you problems. You don't have to "ease back into it" or anything like that.
The only thing that you'd need to do differently at this point is prepare yourself mentally (basically resign yourself to the fact that you can't be vegetarian like you want to be) and choose the seafood that you most enjoy eating (to cut back on any taste/texture unpleasantness). Obviously, don't start with something deep-fried (too much greasy food will make you sick no matter what's inside the breading).
If you'd gone several months (say, six or more) without eating meat, you would probably have to start a bit more slowly, but one month really isn't enough for any sort of physiological change to occur. After a long time without eating a particular food, you will produce less of the enzymes required to digest it, but your body will start producing them again once you begin to eat meat. It does take awhile to get back up to "normal" levels, so it's not a great idea to eat a giant steak as your first non-veg meal. But since you've only been avoiding meat for a month, chances are you could eat a regular (not restaurant) serving size without problems.