If she isn't puking up the whole feed and isn't bothered by it then really it's just a laundry issue
She will grow out of it, when she's more upright for longer periods and on solids you will probably find it gets a lot better (around 6 months ish). Also as long as she is gaining weight well and sticking to her growth curve then she isn't loosing too much!
I'm assuming you are in the UK, so if you did want to try thickeneing her feeds your options are to buy a thickener called Carobel or try the reflux fomula. Both are only available from a pharmacy and would probably need to be ordered in. My two did really well on Enfamil AR which is a cow's milk based formula that has added rice starch, it thickens in the stomach so it's not refluxed back up. SMA do a reflux formula called Stay Down.
If you did want to try the Carobel or reflux formula make sure you speak to your HV or Dr first. Really though, if you can wait it out and it will improve!
Laura