Tubes tied at 21?

I'm almost 28 weeks with my 3rd child, (a boy!) and I'm really having a hard time deciding if I want to get my tubes tied. I have to sign the paper 30 days before delivery so I'm feeling pressured somewhat... I love kids and I've always wanted 5, a big family, since I have a very very small family. But then again is that right? I'm a single mother but I feel eventually I will find "the one" and will want more kids. IDK.