These can be symptoms of pregnancy but they can also be symptoms which mean you're getting your period soon, you're eating more than normal (bigger stomach), your posture is bad (back ache) and they can be symptoms of stress. You're reading too much into everything. My advice is to take a pregnancy test to reassure yourself. If you can't do this, you need to calm down or talk to a trusted adult. We have had this question many times before and nobody has never been pregnant. I'm not sure why you are assuming your brother left sperm on a towel and you would know if you inserted this part of the towel into your vagina.
Please read through our site (put Sex 101 into our search engine). You are not understanding how biology and pregnancy works, therefore you think you could be pregnant this way. You can't.