First off are you sure you ovulated? Ovulation can often envoke the same symptoms as pregnancy. You haven't felt an ovulation in 4 years and in that 4 years your body has changed although your period has not.
See stating that your cycle is every 28 days while on the pill is misleading. Those pills keep you regular, and going off of them may sometimes make you ovulate the first month, but more often than not you don't ovulate that first month off the pills. It may take 45 days before your body realizes it has to send the hormones to make you ovulate rather than having a pill introduce those chemicals that keep you from ovulating.
Another thing you don't realize while you're on the pill, stress can mess with your cycle. See, now, you have to pay attention. You have a lot going on in your life. If the schedule has you stressed or you're not eating right because of the schedule then your body uses the extra energy it would spend on regulating your ovulation on handling the stress.
I can offer you one glimmer of hope, it's more often that you get a false negative with a pregnancy test than a false positive. However if you switch brands and it's still negative well, you probably didn't ovulate.
I didn't ovulate right for years. Sometimes I could go for months then boom, I was throwing up and having all the symptoms of pregnancy 2 weeks before even a blood test could confirm I was pregnant. It wasn't until I was diagnosed with PCOS that I realized that was Ovulation symptoms. I still get them every month. I know I'm not pregnant because my husband is sterile, thanks to his vastectomy in 2005.
If you're looking to get pregnant, do all you can to make sure your body is healthy enough for the pregnancy. Eat right, stay away from smoking, no alcohol and excercise. If you have any doubts about this, go have blood tests done. Make sure there's not a chemical imbalance preventing you from ovulating like low thyroid, high insulin, or low estrogen. Get them to do it all.