Sorry you have had such a difficult start to BF, babyblues.
The overwhelmingly most likely reason for a drop in your supply is NOT tongue tie, lack of fruit in your diet (that one is ridiculous, sorry), lack of oats, your period starting (not at two weeks post natal)....it's pure and simple lack of effective and frequent removal of milk from the breast.
If you have been expressing since the start and not BF direct the ONLY way to build up and maintain production is to express at least eight times in 24 hours, and this should be day and night.
If you don't do this it is very hard to keep up a supply.
However, it's not clear from your post if your baby is BF direct as well and you are not using formula. If she is BF direct and you are expressing as well. , it may be that she is getting what she needs and your breasts are simply behaving normally! In the very early days mothers tend to over produce - few women would manage to keep up six ounces in ten minutes....that's very generous even in the very early days scenario.
Yes, speak to someone expert in real life. There's not enough info in your post to know what's happening. Hope it works out and post again with more details if you can.