You and Sasha have been seeing each other for a month. Sasha has been clear from the start: sex only with a condom. You agreed.
Tonight things are heading that way, and you realise you have run out.
The principle
Consent can come with conditions, and the conditions are part of it. If you change what was agreed without the other person knowing, they have not agreed.
If this brings something up for you
You can stop at any time. These UK services are free and confidential. In an emergency, call 999.