If this is a Schedule C business it is a disregarded entity and it's all yours personally. Just like another bank account. Doesn't matter where the money comes from or goes. It's just moving from one pocket to another. What do you mean by "reinvest"? You just deduct any actual expenses you paid, no matter where the money came from (like from your checking account or credit card). So you can even spend more than the income the business received.
Here is some IRS reading material……
IRS information on Self Employment
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center
Pulication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf
Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf
