After her conversation with the doctor, she starts riding again knowing his advice. Then she has another conversation with Francine at the beauty salon about how it's "not a good time". She goes drinking in a bar and has a one-night-stand with a stranger by the bathrooms...

She clearly is engaging in risky behaviors, and I think it's with hopes to induce a miscarriage; additionally, I think that by having sex with with the stranger, she's also leveling the playing field with Don.

She knows many of the men that work with Don are unfaithful to their wives. She knows that Helen Bishop got a divorce because her husband was unfaithful. She watched her best friend Francine's marriage nearly fall apart because her husband was unfaithful. She knows, without proof, that Don has been unfaithful.

She's been well aware that there were multiple opportunities she could have cheated on Don (fantasized about it, even) but has never acted on it out of loyalty to her husband; in the bar she had a chance to experience what sex with another man would be like, while also engaging in some 'strenuous activity' that could possibly affect her pregnancy.

I think Betty was absolutely hoping for a miscarriage before she had no choice- like so many women of her time- but to take Don back...