Welcome to MDC!! 
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Diet can absolutely affect your fertility. But I seriously doubt that the odd diet coke and glass of wine are doing anything. What plays in more to fertility is your weight and the nutrients that you are getting. For example, a lot of vegetarians are low on vitamins like B complex, and could do with a supplement of B6 and B12 especially. Making sure that you are not over or under weight is important as estrogen and progesterone are both stored in fat. So taking a look at your caloric and fat intake as well as your exercise routine couldn't hurt.
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But honestly, you're still breastfeeding and that is probably the biggest factor affecting your fertility at this point. Even though my cycles resumed at 8 months postpartum with my first, they were not fertile again until my dd cut way back on her nursing and night weaned a full 2 years postpartum. So it took my cycles 16 months to regulate once they resumed. Every woman is different.
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To find out what is happening with your cycles, I highly recommend charting your basal body temperature and cervical fluid. This will tell you when you are fertile, if/when you ovulate, how long your luteal phase is (the time between O and AF- it is generally agreed that you need at least 10 non-spotting days to sustain a pregnancy), when to test, when to expect AF, if you're pregnant and if there is anything of concern happening with your cycle. If you'd like more information about charting, just post! Good luck!