Well I'm going to disagree slightly with the consensus (just because I can) 
First, there is nothing wrong with being pregnant at 21. Your relative youth means that you are less likely to be in a reasonable financial position, but plenty of older people also have kids in poor financial situations. I can't see the difference.
Second, PROVIDING you did not mislead the father, and you didn't lie and say you were on contraception, then he was just as cavalier as you. Sleeping around is dodgy from an STI point of view, but that aside I can't see how it's different to another woman deciding she wants to have a child and going to a sperm bank. This is something which many posters on MN often suggest as an option for women in maybe their late 30s who don't have a partner but who want kids.
That said, I would not want to be in your position myself. I waited until I was in a loving marriage before creating children, both for my own benefit and for theirs and for their father's. This is the life I want. If my husband dies I will manage of course, but I would not choose single parenthood.
But it's done now. In your shoes I'd make a financial plan first before talking to your mum. Demonstrate you've thought this through (I assume you have). If you're working just now you'll get some maternity pay for a bit off the government, but what's your plan after 9 months when it runs out? Do you earn enough money to cover childcare? Whilst it may be that your mum might offer to do a day a week at a later stage, don't whatever you do factor this into your plan! Will the father pay maintenance? Find out how much that will be.
Once you've done that, tell your mum and tell her your plan. Yes, she will probably think you've been far too hasty. She will probably be really sad because you could have done it in a more traditional way and you will miss out on a lot because of this. Give her a few days/couple of weeks to be sad/disappointed. After that, hopefully she will support you even if she doesn't agree with your decision. Good luck!