Has My Home Internet Been Throttled?

I think my home Internet service has been throttled but I cannot confirm this. I’ve run speed tests numerous times and only been able to get a max of 1.5 Mbps on the download side. This is quite low since I am paying for the 6Mbps service. I got a warning email from AT&T reminding me that I am limited to 150GB per month.

When I open the link from this email it takes me to a page with helpful information about what 150GB means.

There’s also a link to show my usage since my last page that does not actually work.