There's nothing wrong with reaching up high - I think the reason some people feel weird about it is they think they're 'stretching' their uterus and are going to detatch the baby or some such idea! (my GP even told me not to do freestyle swimming because too much reaching up was bad, but both my physio, who specialises in obstetric physiotherapy and my obs have said its all rubbish and reach as much as I want!)

Having said that, be careful when lifting heavy things high, because that can put strain on your lower back, and your abs are unfortunately not in any state to support your back at this stage! I haven't heard about the "arms above your head makes you dizzy" thing, but it could be true - so if it happens to you, don't do it!