Spouse lies about spending time with a girl, financially supports her. Indication of physical relationship?

His explanation doesn't make a possibility mutually exclusive. Things can seem a certain way strongly as long as you're speculating. The fact that your husband stopped supporting her financially after you objected indicates a different outcome but the fact that he lied to you about this situation indicates otherwise.
I think voicing your displeasure about his actions more articulately would serve as a step to setting boundaries in your marriage.
The next thing to consider talking to him about would be his expectations from your marriage. It maybe possible that because he feels that your marriage has not provided him with his necessities ( secure space and bond, ability to voice opinions, rely on you in times of need etc), leading to him looking outside for a bond and indulging in a female colleague's time.

There are endless possibilities as to why you're husband initiated a bond that you may not approve off. The only way to know would be to talk to your husband objectively. The speculation will only cause more distress and distance between the two of you.

The conversation will also help understand your husband's view of his actions. Whether he wants to rebuild your trust in the relationship or is not guilty of having risked his marriage with you, only then the two of you can obtain an idea of what needs to be done to avoid the constant speculation.

It is laudable that you reached out to ask for help to better understand your marriage and spouse. It may also help you both to seek marriage counselling to get past the incident and rebuild your marriage.