Dating

How do you know if a girl is developing feelings?

Been seeing this girl since February. After about a month, she said she didn't have feelings for me, but was interested in exploring the possibility of a relationship with me. Three months later, and we've been in a casual relationship. We've cuddled, kissed, made out, go on dates weekly, been exclusive, the whole shabang. I'm satisfied with as much she's given me, even if it means not being able to call her my girlfriend. Her not having feelings isn't a big deal.

However, things started feeling different. In May, we went three weeks without seeing each other because of us being busy. She started saying she missed me, started calling me cute, flirting, and being a little clingy. Understandable, because three weeks is the longest we haven't seen each other. Thing is though, we saw each other last Monday and then again on Saturday, and she's acting like this even more. She's been telling me she misses me everyday and wants to know what I've been up to. she's also been very flirty, and tells me she wishes I was with her physically. We have a weekly phone call every Sunday, and she tells me all the things she's doing during the week unsolicited. Very clingy. When we're together I feel like I need a stick to keep her off of me. She's started talking about marriage a lot as well, where before she said she didn't want to get married, period. I fully expected all this to stop when our hiatus was over, and things would go back to normal.

Now I'm not assuming she has feelings, because she's only ever said the opposite. However something in my gut is telling me she is developing feelings, and it's a feeling I can't shake.
How do you know if a girl is developing feelings?
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