I think the way it is is fine. I didn't know there was a flashback going on and was like "Oh, she's so short" but then in the next update was able to get it easily. I don't think you have to spoonfeed readers that it is a flashback, especially with it being so short and at the beginning of the comic. I don't think it would work if it were any longer than that and it's a technique I think should be used sparingly.
I like the style of the sketchiness but it does feel a bit unfinished. Not necessarily because of the sketchiness, but sometimes it looks a bit messy and maybe could be cleaned up a bit more? I've attached an example:
I love your style, especially the hair! Story looks like it will be interesting so I have subscribed.
edit: And definitely frame borders like @jensrichard77 mentioned would be helpful!