They only cost like 40c each. You could use one every day from 8 dpo and still have enough for several cycles.
The older you are the harder it is to lose / maintain your weight.
But you seem to be saying you are worried about pregnancy? If so, just take a home pregnancy test. They are cheap and easy, so there is no reason not to. Example of how cheap they are: http://www.amazon.com/Wondfo-Pregnancy-Test-Strips-50-count/dp/B004AOMAOG
DO NOT WATCH THAT SHOW.
I have been where you are and it SUCKS. In fact I still occasionally panic about pregnancy even though my husband has had a vasectomy. Seriously.
Like many other anxiety-based problems, there's a huge difference between KNOWING something and BELIEVING it. You know you're not pregnant. You used protection, you got your period, you understand how basic biology works. You know you're fine. But anxiety sometimes prevents us from accepting facts.
What worked for me was a multi-step approach. First, I started tracking my periods so instead of guessing when I was late I would know. There are tons of free apps for this and it's super easy to track. Next, I got on birth control (a huge hurdle since I can't take estrogen because of migraines with aura). Finally, I got a 50 pack of pregnancy test strips from Amazon. Since I was on the mini pill my periods were a little irregular, so I made the rule that I would not test until I was at least three days late. The strips are cheap ($19 for 50), easy to use, and very accurate.
Taking those steps didn't completely solve my pregnancy related hypochondria (pregochondria!) but they certainly helped. From time to time I still worry about accidentally getting pregnant. And again: my husband had a vasectomy, so it's a pretty damn irrational worry- but I don't freak out the way I used to back when I wasn't tracking and we only used condoms.
They work great. There are also smaller/cheaper bundles if you don't want to buy 50.
Might as well post this here...pregnancy tests in stores are ripoffs, if you are a sexually active female, pick up a 50-pack of pregnancy test strips for $16 from Amazon. They stay good for like 2 years.
For the future, if you're worried about being pregnant, try these pregnancy tests (I'm not the seller): http://www.amazon.com/Wondfo-Pregnancy-Test-Strips-50-count/dp/B004AOMAOG/ref=sr_1_4?ie=UTF8&qid=1311182944&sr=8-4
Someone recommended them to me on r/twoxchromosomes after discussing birth control/no condom sex (which I am NOT recommending to you at such a young age and for someone at the beginning of their sexual activity and not on the pill). The test is really accurate and super cheap.
Constantly. I bought a pack of like 200 strip tests on Amazon bc I test at least twice a month. It's too expensive to buy the ones at the pharmacy.
Edit: These https://www.amazon.com/dp/B004AOMAOG/ref=cm_sw_r_other_apa_uvsoBb90DZ0FE
Or not? I bought these a while ago and it's just "Amazon Marketplace" on my statement.