cup causing very long periods?

Hi everyone! 
This is my first post here but I've read come here before and read posts about lots of different things! I'm wondering if anyone has experienced longer/heavier periods due to a cup? This is my third cycle using a small diva cup. I really love it! I've always had a wonky cycle (it would come for 2 months then leave for 5. I'm about 15 and a half, a virgin, and have had my period for 2 years as of July) Anyway I first tried a cup in April and loved it. I do not recall anything odd in this cycle.
When I got my period in May I tried it again and loved it but found that by about day 3 my flow got really heavy, there was a time where in about 70 mins I had overflowed the cup (and it was overflowed, not just leaking! Not fun, especially when I was at the lake and we're not supposed to flush every time.) Anyway, it lasted about 8 days and got better throughout but I had to wear pads for most of it.
In June I got my period halfway through the month and have now had it for 25 days. (luckily the cup makes it like it's not even happening, so awesome!) I don't have cramps or anything just flow. In about 8 hours it's just up to the first line (can't remember the measurement!)
I was talking to my cousin and she suggested maybe the cup was to blame. Has anyone ever heard of a cup causing periods to be a lot longer and/or heavier? I looked online and haven't found anything. I am going to the doctor in a few days but I was wondering if anyone on here knows anything. Thank you so much!!!