Runner/Foot Question!

RunEloiseRun

Hi y'all.
I am currently in the midst of training for my first marathon on Oct. 7th and I think I may have an injury.
For the first five and a half weeks of training, I was purely running on the balls of my feet. I thought it was the best way to run and I ended up getting blood blisters on the inside of my right foot. I changed the way I was running once I saw the blisters and did some google-ing about them. I stopped running on the balls of my feet and started focusing on running the way I had previously ran the year before.
I was still getting terrible blisters so I decided to get new shoes. (Mine were pretty worn at that point.) I was wearing Mizuno Wave Rider 15s (a neutral shoe, even though all of last year I wore Wave Inspire 7s a stability shoe) and now I have been running in Wave Inspire 8s. I have no more blister problems, but now I have this achy feeling in my left foot. I have worn my WI8s on two runs now. The first run was 7.40 miles and the second run was 3 miles. The pain in my left foot started in the middle of the 4th mile on the first run in my new shoes. The pain is right in the curve of my foot before my heel. Not in the middle of the arch, a little lower, but again not in the heel.
I am not sure if it's the shoes causing this, the running itself, or something else.

Last year I ran a half marathon and experienced no problems.
I did gain back some weight this year and I did go a stretch without running....I just do not want to be sidelined for too long nor do I want to be plagued with this pain or make it a worse problem.

Any help, suggestions, or advice would be greatly appreciated!