Typically, when you join a company you're employed as an engineer to work with whatever technologies and systems they use regardless of whether you have any previous experience with it or not. Companies know when they take on juniors, that those people are most likely going to need to spend considerable time learning and making mistakes. I expect your manager has employed you with the hope that you're a fast learner; I doubt they took you on to be a specialist in one particular technology which you already knew beforehand.
However, unless the developers around you had also been part of your interview/selection process I expect those people have no idea exactly what your skills/experience are, they're probably just treating you as another junior member of the team and sharing out the workload, assuming that you've got at least a little knowledge to get started with, then learning the rest as you go .
All senior developers were juniors once, and most likely wouldn't deliberately give you anything which they thought you'd not be able to cope with (As seniors, they'll be aware that the blame will fall upon them if they've not been keeping you in check). Best thing is to be open about it as early as possible to them and to your direct manager/supervisor. The project managers will be interested in the work getting done on time, and someone will do something about it if they realise that you're unable to meet those deadlines on your own (However if you leave it so long that the deadlines get missed, then you'll be taking the brunt of the blame - and you definitely don't want to get into that situation!).
You need to be honest about it to everyone you work with/for, and then treat it as a fast-paced learning experience. Someone in the company will have the job of managing the engineers, and it will be in their best interests to find a way to help you cope (even if that ends up being a steep learning curve for you, you'll hopefully benefit from it in the long run).