Pyrus Calleryana Chanticleer - Struggling?

Hi all, I have a problem with my tree with the leaf stems turning red...

I planted an Ornamental Pear, which I bought from the ornamental-trees website, at the end of February this year (2020). Everything started off well and by the end of April, there were leaves all over it, looking a uniform deep green colour.

At the beginning of May (3rd), I applied a couple of handfuls of NPK 7-7-7 fertiliser.
Coincidentally, on the 5th May, I noticed that some of the leaves looked more pale in colour and on closer inspection, realised that insects had been at quite a few of them. Here are a few pictures:

Any ideas on what insects are doing this and if I need to worry? (Keep in mind that the stems and veins are still green at this stage.)

Over the past few days, I've noticed that a lot of the stems and leaves have started turning red. Even the new leaves that are coming out look red:

What do you think is going on?
This is the first time I've ever planted a tree and I hope I haven't done anything to cause this. Is there anything I need to do to help the tree at this stage?
I give it about 5L of water every 2 days.

Hope you guys can help.

Thanks,

Tree-Noob!