Why is my mobile internet getting slower?

So about a week ago I got a new mobile phone upgrade on my contract (Sony Xperia Z). Well a couple days ago I was getting on average 20mbps (highest 27, lowest 13) but the past day I haven't gotten over 2mbps. What's wrong. Will I have to ask my provider or is this normal for a new phone even if it's the same sim and network. BTW I'm on the three network in the UK. Oh and my old phone was getting on average 3.5mbps (NEVER got over 6mbps) I had the iPhone 4 if it makes any difference. Thanks in advance!

Added (1). Or are the cell towers down or something?

I think your cellphone is one of the top advanced smrtphones in the world and there's nothing wrong with it! I think contactting your provider or carrier ir a better choice?! Good hope!

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