Ovulation question

Is it for certain you ovulate exactly 2 weeks after your last period started? My last started on August 28th and has been off by 2 to 4 days. My cycle has always been 28 days but not since Jan/Feb. One app says I ovulated on the 11th and Premom app said the same until I got a OPK peek on the 8th. Now I’m confused as to when I actually ovulated 😩 I took a test this morning with a pregnancy test strip and got a BFN! I’m hoping it’s too early, maybe I did ovulate late 🤷🏼‍♀️ Thanks for any input ladies. Trying to stay positive. We’ve been trying since April to get pregnant.