Have you changed phones? Some years ago T-Mobile started calling HSPA+ (Which is 3.5G) "4G Speed", then they just dropped the "Speed" and called it "4G". theis presented a bit of a problem when real 4G (LTE) came along and they had to come up with a different name because "4G" was in use. They came up with "4G LTE". However, some non-T-Mobile phones report LTE as just "4G" and HSPA+ as 3.5G or 3G or "H".
If you have not changed your phone or location, it may be that LTE is highly congested and the network has shifted you off to HSPA+ to provide better service.
Let us know which device you're using and what steps you have taken to fix the issue.
Just checking in here to see how things are going. Please let us know if you still have questions.