you cannot get pregnant by swallowing sperm. however, you can pass around a host of diseases not limited to HIV, gonorrhea (it can infect your throat, and that's never pretty), herpes (1 in 3 persons has oral herpes and probably didn't get it via sex, but can pass it to sexual partners' genitals during oral sex) and a few others.
use a condom during blow jobs. a little lube on the inside and it'll still feel great and it's safer for the both of you.
as for pregnancy symptoms, the only real, surefire way to know if you're pregnant is to take a home preg. test. backaches and nausea can be caused by bad chili.
and the way a woman gets pregnant is by getting sperm onto her genitals and into her reproductive tract. swallowing sperm won't do it unless you drooled so much that it got all over your vulva or something.
anyway, it's a good idea to get screened for STDs (why would you ever want to cut corners about oyur health?). Then get some condoms and flavored lube.
here are a bunch of threads about fun, safety and oral sex. if you read them, they'll help you out bunches.
http://www.scarleteen.com/forum/Forum21/HTML/000058.html http://www.scarleteen.com/forum/Forum21/HTML/000009.html http://www.scarleteen.com/forum/Forum21/HTML/000045.html
Correlation does not equal causation.