How can I tell if my boyfriend is seeing someone else?

So my boyfriend has been really lovey lately. When I mean lovey is like he's always bringing me flowers hugging me, wanting to spend money on me but when I ask him why are you doing all this he goes quiet then gets mad. I always have that sick feeling in my gut that he is ... But also he won't give me his phone but if I do ask for it it takes him 5mins to give it to me? He's been 'working' a lot lately... Advice please xx ❤️❤️